ODD Days
Are the days which are more than the complete number of weeks in a given period.

Leap year:
Every year which is divisible by 4 is called a leap year. For Eg: 1992,1996,2000,2004
Every 4th century is a leap year but no other century is a leap year Eg: 400,800,1200
Years other than the leap years are called as Ordinary / Normal years
An Ordinary year contains 365 days, 52 weeks and 1 ODD day
A Leap year contains 366 days, 52 week and 2 odd days.

Calculation of Odd days:
No. of given days/7(week), answer is the reminder.
Odd days in 45 days
45/7, total weeks are 6  and odd days are 3 (reminder)

Calculation of leap Year:
Odd days in 20 years
These 20 years is a combination of certain leap and ordinary years.
20 years consists of 5 leap years and 15 ordinary years.
Every leap year has 2 odd days and an ordinary year has 1 odd day
5x2 = 10 ; 15x1 =15;  Total =25
It is known that the odd days cannot exceed 6 days.  Hence, 25/7, odd  days are 4.
Counting of days : Sunday for 0 odd day, Monday for 1 odd day and so on.

No. of Weeks in a Ordinary Year (not leap year)

365/7 = 52 weeks + 1 odd day (remainder)

No. of Weeks in a Leap year)

366/7 = 52 weeks + 2 odd days (remainder)