**Important Facts And Formulae**

**Odd Days :**

We are supposed to find the day of the week on a given date.

For this, we use the concept of ‘odd days’.

In a given period, the number of days more than the complete weeks are called odd days.**Leap Year :**

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

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

**Note**: A leap year has 366 days.

Examples :

i. Each of the years 1948, 2004, 1676 etc. is a leap year.

ii. Each of the years 400, 800, 1200, 1600, 2000 etc. is a leap year.

iii. None of the years 2001, 2002, 2003, 2005, 1800, 2100 is a leap year.**Ordinary Year :**

The year which is not a leap year is called an ordinary years. An ordinary year has 365 days.**Counting of Odd Days :**

**1.**1 ordinary year = 365 days = (52 weeks + 1 day.)

1 ordinary year has 1 odd day.

**2.**1 leap year = 366 days = (52 weeks + 2 days)

1 leap year has 2 odd days.

**3.**100 years = 76 ordinary years + 24 leap years

= (76 x 1 + 24 x 2) odd days = 124 odd days.

= (17 weeks + days) 5 odd days.

Number of odd days in 100 years = 5.

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

Number of odd days in 300 years = (5 x 3) 1 odd day.

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

Similarly, each one of 800 years, 1200 years, 1600 years, 2000 years etc. has 0 odd days.

**Day of the Week Related to Odd Days** :