# Important Formula For Calendar – Aptitude Tips

Important Formulas – Calendar

1. Odd Days

Number of days more than the complete weeks are called odd days in a given period.

2. Leap Year

A leap year has 366 days.

In a leap year, the month of February has 29 days.

Every year divisible by 4 is a leap year, if it is not a century.

Examples:

1952, 2008, 1680 etc. are leap years.
1991, 2003 etc. are not leap years

Every 4th century is a leap year and no other century is a leap year.

Examples

400, 800, 1200 etc. are leap years.
100, 200, 1900 etc. are not leap years

3. Ordinary Year

The year which is not a leap year is an ordinary year.

An ordinary year has 365 days

4. Counting odd days and calculating day of any particular date

1 ordinary year ≡ 365 days ≡ (52 weeks + 1 day)
Hence number of odd days in 1 ordinary year= 1.

1 leap year ≡ 366 days ≡ (52 weeks + 2 days)
Hence number of odd days in 1 leap year= 2.

100 years ≡ (76 ordinary years + 24 leap years )
≡ (76 x 1 + 24 x 2) odd days
≡ 124 odd days.
≡ (17 weeks + 5 days)
≡  5 odd days.

Hence number of odd days in 100 years = 5.

Number of odd days in 200 years = (5 x 2) = 10 ≡ 3 odd days

Number of odd days in 300 years = (5 x 3) = 15 ≡ 1 odd days

Number of odd days in 400 years = (5 x 4 + 1) = 21 ≡ 0 odd days

Similarly, the number of odd days in all 4th centuries (400, 800, 1200 etc.) = 0

Mapping of the number of odd day to the day of the week

 Number of  odd days Day of the week 0 Sunday 1 Monday 2 Tuesday 3 Wednesday 4 Thursday 5 Friday 6 Saturday