Day of the week calculator

Yes, for the Gregorian calendar (adopted from 1582 in most of the world).

Yes. The calendar is predictable, so any future date's weekday can be found.

Because a year has 365 days (52 weeks + 1), so a date advances one weekday per year — and two after a Feb 29.

Because 365 days is 52 weeks plus one leftover day, which pushes the date one weekday forward each year — two when a 29 February falls in between.

Every 28 years within a century without a leap-year exception, the calendar repeats identically.

For Gregorian dates, yes. Before October 1582 care is needed: ten days were skipped in the reform, and countries adopted it at different times.