input: Write a python function to find the day of the week for January 1st of a given year.
  assert day_of_week(2018) == "Monday"
  assert day_of_week(2020) == "Wednesday"
  assert day_of_week(2021) == "Friday"
output: def day_of_week(year):
      import datetime
      days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
      first_day = datetime.date(year, 1, 1).weekday()
      return days[first_day]