Return start date of the year using MMWR week rules

def _start_date_of_year(year: int) -> datetime.date:
    """
    Return start date of the year using MMWR week rules
    """

    jan_one = datetime.date(year, 1, 1)
    diff = 7 * (jan_one.isoweekday() > 3) - jan_one.isoweekday()

    return jan_one + datetime.timedelta(days=diff)