pub fn day_of_week(year: i32, month: u8, day: u8) -> u8
Day of the week for (year, month, day). 0 = Sunday, 1 = Monday, …, 6 = Saturday. Uses Zeller’s congruence; undefined for years before AD 1.
(year, month, day)
0 = Sunday
1 = Monday
6 = Saturday