Calendars --Key Points

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Odd Days

The number of days more than the complete number of weeks in a given period is number of odd days during that period.

Leap Year

Every year which is divisible by 4 is called a leap year. Thus each one of the year 1992, 1996, 2004, 2008, 2012, etc. is a leap year.

Every 4^th century is a leap year but no other century is a leap year. Thus each one of 400, 800, 1200, 1600, 2000, etc. is a leap year. None of the 1900, 2010, 2020, 2100, etc. is a leap year. A leap year has 366 days.

Ordinary Year

An year which is not a leap year is called ordinary year. An ordinary year has 365 days.

Counting of odd days

One ordinary year = 365 days = 52 weeks + 1 day. Therefore, an ordinary year has 1 odd day.
One leap year = 366 days = 52 weeks + 2 days. Therefore, a leap year has 2 Odd days.
100 years = 76 ordinary years + 24 leap years = 5217 weeks + 5 days. Therefore, 100 years contain 5 odd days.
200 years contain 10 (1 week + 3 days), i.e 3 odd days.
300 years contain 15 (2 weeks +1 day), i.e 1 odd day
400 years contain (20 + 1), i.e 3 weeks, so 0 odd days. Similarly, each one of 800, 1200, 1600, etc. contains 0 odd days.
Note: (7n + m) odd days, where m less than or equal to 7 is equivalent to m odd days.

Week 

• Sunday - 0
• Monday - 1
• Tuesday - 2
• Wednesday - 3
• Thursday - 4
• Friday - 5
• Saturday - 6