Without Calendar :
Day-of-week quick recall from memory. See this chart? All the dates in this calendar fall on the same day of week every year repeatedly. And for this year that day is Saturday. Every new year day-of-week skips forward by 1 day and thus for 2021 it will be Sunday, while the dates on the calendar keep exactly the same. For leap year, skip one day-of-week more as soon as the leap day passes but keep the dates as is always.
So this basically is the perpetual calendar, which once you memorize, you can work out day of week from date, or date from day of week for the rest of your life.
Search "Doomsday Algorithm".
