The entire calendar concept revolves around 'Odd Days'. Every 7 days, the days of the week repeat. Odd days are the remainder when total days are divided by 7.
Key Formulas
Leap Year Rule
Year % 4 == 0 (If century year, it must be divisible by 400)
2000 is a leap year, 1900 is not.
Odd Days in 100 Years
5 odd days
200 years = 3 odd days. 300 years = 1. 400 years = 0.
⚡ Shortcut Tricks
#1
Ordinary year has exactly 1 odd day.
#2
Leap year has exactly 2 odd days.
#3
If a year starts on Monday, an ordinary year ends on Monday, while a leap year ends on Tuesday.
#4
Calendars repeat completely when the sum of odd days between them becomes a multiple of 7.