Return this or next Monday.

find_next_monday.py

#!/usr/bin/env python3
from datetime import date, timedelta


def find_next_monday(yyyymmdd):
    """Return this or next Monday.
    """
    date_iso = date.fromisoformat(yyyymmdd)
    day_o_wk = date_iso.isocalendar().weekday  # Mon: 1, ... Sun: 7
    dy_delta = 8 - day_o_wk
    next_mon = date_iso + timedelta(days=dy_delta)
    return date_iso if day_o_wk == 1 else next_mon


if __name__ == '__main__':
    mon = find_next_monday(input('Date? (ISO format) >> ')).strftime('%a, %Y-%m-%d')
    print(f'Next(or this) Monday is {mon}')