from tkinter import *
from tkcalendar import Calendar
root = Tk()
root.geometry("400x400")
cal = Calendar(root, selectmode='day',
               year=2020, month=5,
               day=22)
cal.pack(pady=20)
def grad_date():
    date.config(text="Selected Date is: " + cal.get_date())
Button(root, text="Get Date",
       command=grad_date).pack(pady=20)
date = Label(root, text="")
date.pack(pady=20)
root.mainloop()

Fonte do Código