AttributeError: 'Messenger' object has no attribute 'pushButtondateadd'

НИКОГДА !!! Не изменяйте код, сгенерированный Qt Designer. Создайте другой класс, который наследуется от соответствующего виджета, и используйте созданный класс для его заполнения.

import requests
from PyQt5 import QtCore, QtGui, QtWidgets


#from login import Login
#class Login(QtWidgets.QDialog):  
class Ui_Login(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 182)
        self.label = QtWidgets.QLabel(Dialog)
        self.label.setGeometry(QtCore.QRect(0, 15, 401, 31))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label.setFont(font)
        self.label.setTextFormat(QtCore.Qt.RichText)
        self.label.setScaledContents(False)
        self.label.setAlignment(QtCore.Qt.AlignCenter)
        self.label.setTextInteractionFlags(QtCore.Qt.NoTextInteraction)
        self.label.setObjectName("label")
        self.lineEditLogin = QtWidgets.QLineEdit(Dialog)
        self.lineEditLogin.setGeometry(QtCore.QRect(210, 70, 141, 21))
        self.lineEditLogin.setObjectName("lineEditLogin")
        self.label_2 = QtWidgets.QLabel(Dialog)
        self.label_2.setGeometry(QtCore.QRect(60, 70, 161, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_2.setFont(font)
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(Dialog)
        self.label_3.setGeometry(QtCore.QRect(50, 110, 191, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_3.setFont(font)
        self.label_3.setObjectName("label_3")
        self.lineEditPassword = QtWidgets.QLineEdit(Dialog)
        self.lineEditPassword.setGeometry(QtCore.QRect(210, 110, 141, 21))
        self.lineEditPassword.setObjectName("lineEditPassword")
        self.loginbutton = QtWidgets.QPushButton(Dialog)
        self.loginbutton.setGeometry(QtCore.QRect(160, 150, 91, 23))
        self.loginbutton.setObjectName("loginbutton")

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.label.setText(_translate("Dialog", "Мессенджер школы №7"))
        self.label_2.setText(_translate("Dialog", "Введите логин:"))
        self.label_3.setText(_translate("Dialog", "Введите пароль:"))
        self.loginbutton.setText(_translate("Dialog", "Авторизоваться"))


#from date import Date
#class Date(QtWidgets.QDialog):
#    def __init__(self):
#        super().__init__()
#        self.setupUi(self)
class Ui_Date(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 346)
        self.label = QtWidgets.QLabel(Dialog)
        self.label.setGeometry(QtCore.QRect(0, 10, 401, 41))
        font = QtGui.QFont()
        font.setPointSize(20)
        self.label.setFont(font)
        self.label.setTextFormat(QtCore.Qt.RichText)
        self.label.setScaledContents(False)
        self.label.setAlignment(QtCore.Qt.AlignCenter)
        self.label.setTextInteractionFlags(QtCore.Qt.NoTextInteraction)
        self.label.setObjectName("label")
        self.label_2 = QtWidgets.QLabel(Dialog)
        self.label_2.setGeometry(QtCore.QRect(40, 60, 151, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_2.setFont(font)
        self.label_2.setObjectName("label_2")
        self.datename = QtWidgets.QLineEdit(Dialog)
        self.datename.setGeometry(QtCore.QRect(150, 60, 211, 21))
        self.datename.setObjectName("datename")
        self.label_3 = QtWidgets.QLabel(Dialog)
        self.label_3.setGeometry(QtCore.QRect(80, 100, 151, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_3.setFont(font)
        self.label_3.setObjectName("label_3")
        self.datedate = QtWidgets.QDateEdit(Dialog)
        self.datedate.setGeometry(QtCore.QRect(150, 100, 110, 22))
        self.datedate.setObjectName("datedate")
        self.datetime = QtWidgets.QTimeEdit(Dialog)
        self.datetime.setGeometry(QtCore.QRect(150, 140, 118, 22))
        self.datetime.setObjectName("datetime")
        self.label_4 = QtWidgets.QLabel(Dialog)
        self.label_4.setGeometry(QtCore.QRect(70, 140, 151, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_4.setFont(font)
        self.label_4.setObjectName("label_4")
        self.label_5 = QtWidgets.QLabel(Dialog)
        self.label_5.setGeometry(QtCore.QRect(40, 180, 151, 21))
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_5.setFont(font)
        self.label_5.setObjectName("label_5")
        self.dateinfo = QtWidgets.QTextEdit(Dialog)
        self.dateinfo.setGeometry(QtCore.QRect(150, 180, 211, 101))
        self.dateinfo.setObjectName("dateinfo")
        self.pushButtonAdd = QtWidgets.QPushButton(Dialog)
        self.pushButtonAdd.setGeometry(QtCore.QRect(260, 300, 91, 23))
        self.pushButtonAdd.setObjectName("pushButtonAdd")
        self.pushButtonCancel = QtWidgets.QPushButton(Dialog)
        self.pushButtonCancel.setGeometry(QtCore.QRect(150, 300, 91, 23))
        self.pushButtonCancel.setObjectName("pushButtonCancel")

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.label.setText(_translate("Dialog", "Создание события"))
        self.label_2.setText(_translate("Dialog", "Название:"))
        self.label_3.setText(_translate("Dialog", "Дата:"))
        self.label_4.setText(_translate("Dialog", "Время:"))
        self.label_5.setText(_translate("Dialog", "Описание:"))
        self.pushButtonAdd.setText(_translate("Dialog", "Добавить"))
        self.pushButtonCancel.setText(_translate("Dialog", "Отмена"))

#from main import Main
#class Main(QtWidgets.QMainWindow, Date):
#    def __init__(self):
#        super().__init__()
#        self.setupUi(self)
class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(1920, 1080)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.dialogs = QtWidgets.QListWidget(self.centralwidget)
        self.dialogs.setGeometry(QtCore.QRect(10, 10, 221, 491))
        self.dialogs.setObjectName("dialogs")
        self.groupBox = QtWidgets.QGroupBox(self.centralwidget)
        self.groupBox.setGeometry(QtCore.QRect(520, 10, 331, 491))
        self.groupBox.setObjectName("groupBox")
        self.calendarWidget = QtWidgets.QCalendarWidget(self.groupBox)
        self.calendarWidget.setGeometry(QtCore.QRect(0, 40, 331, 191))
        self.calendarWidget.setObjectName("calendarWidget")
        self.lineEdit = QtWidgets.QLineEdit(self.groupBox)
        self.lineEdit.setEnabled(True)
        self.lineEdit.setGeometry(QtCore.QRect(0, 0, 331, 31))
        font = QtGui.QFont()
        font.setPointSize(11)
        font.setBold(True)
        font.setWeight(75)
        self.lineEdit.setFont(font)
        self.lineEdit.setMouseTracking(False)
        self.lineEdit.setTabletTracking(False)
        self.lineEdit.setFocusPolicy(QtCore.Qt.NoFocus)
        self.lineEdit.setContextMenuPolicy(QtCore.Qt.NoContextMenu)
        self.lineEdit.setMaxLength(2048)
        self.lineEdit.setFrame(True)
        self.lineEdit.setAlignment(QtCore.Qt.AlignCenter)
        self.lineEdit.setPlaceholderText("")
        self.lineEdit.setClearButtonEnabled(False)
        self.lineEdit.setObjectName("lineEdit")
        self.pushButtondateadd = QtWidgets.QPushButton(self.groupBox)
        self.pushButtondateadd.setGeometry(QtCore.QRect(0, 240, 331, 21))
        self.pushButtondateadd.setObjectName("pushButtondateadd")
        self.dateinfo = QtWidgets.QTextBrowser(self.groupBox)
        self.dateinfo.setGeometry(QtCore.QRect(0, 270, 331, 221))
        self.dateinfo.setObjectName("dateinfo")
        self.textBrowser = QtWidgets.QTextBrowser(self.centralwidget)
        self.textBrowser.setGeometry(QtCore.QRect(250, 10, 251, 441))
        font = QtGui.QFont()
        font.setStrikeOut(True)
        self.textBrowser.setFont(font)
        self.textBrowser.setObjectName("textBrowser")
        self.message = QtWidgets.QTextEdit(self.centralwidget)
        self.message.setGeometry(QtCore.QRect(250, 460, 161, 41))
        self.message.setObjectName("message")
        self.sendButton = QtWidgets.QPushButton(self.centralwidget)
        self.sendButton.setGeometry(QtCore.QRect(420, 470, 81, 23))
        self.sendButton.setObjectName("sendButton")
        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.groupBox.setTitle(_translate("MainWindow", "GroupBox"))
        self.lineEdit.setText(_translate("MainWindow", "Календарь событий"))
        self.pushButtondateadd.setText(_translate("MainWindow", "Добавить событие"))
        self.sendButton.setText(_translate("MainWindow", "Отправить"))        

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
class Login(QtWidgets.QDialog, Ui_Login):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        
class Login(QtWidgets.QDialog, Ui_Login):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

class FormDate(QtWidgets.QDialog, Ui_Date):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^        

# !!! Множественное наследование применяйте только если вы понимаете что делаете. !!!
#class Messenger(Login, Main, Date):
class Messenger(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        
#        self.loginbutton.pressed.connect(self.login)
#        self.pushButtondateadd.pressed.connect(self.date)

        self.pushButtondateadd.clicked.connect(self._date)             # clicked
        
# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv        
        self.form_login = Login()
        self.form_login.show()
        self.form_login.loginbutton.clicked.connect(self.login)
        
        self.form_date = FormDate()
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    def _date(self):
#        window3.show()
        self.form_date.show()

    def login(self):
        name = self.form_login.lineEditLogin.text()
        password = self.form_login.lineEditPassword.text()
        user_data = {'name': name, 'password': password}
#!        response = requests.post('http://127.0.0.1:5000/login', json=user_data)
#!        if response.status_code != 200:
        if name != 'name' or password != 'password':
            print('Проверка не пройдена')
            msg = QtWidgets.QMessageBox.information(
                self, 
                "Внимание!", 
                "Что-то пошло не так, попробуйте еще раз."
            )
        else:
            self.form_login.hide()
            self.show()


if __name__ == '__main__': 
    import sys
    app = QtWidgets.QApplication(sys.argv)
    window1 = Messenger()
#    window2 = Main()
#    window3 = Date()
##    window1.show()
    sys.exit(app.exec_())