The formula is:

(Year Code + Month Code + Century Code + Date Number – Leap Year Code) mod 7

As a year contains 365 days with factors of 7(1week) gives 1(day) as a reminder which adjusts one day extra to the next year.

Given year is divided by 4, and the quotient gives the number of leap years. But, as 100,200 and 300 are not leap years => 75 – 3= 72 leap years

First,we count the number of odd days for the left over days in the given period.

Here,given period is 12.2.1986 to 1.1.1987

Feb Mar Apr May June July  Aug Sept Oct Nov Dec Jan

16   31    30  31   30    31   31   30   31  30   31   1 (left days)

2 + 3 + 2 + 3 + 2 + 3 + 3 + 2 + 3 + 2 + 3 + 1(odd days) = 1 odd day

So,given day Wednesday + 1 = Thursday is the required result.

Repetition of leap year ===> Add +28 to the Given Year.

Repetition of non leap year

Step 1 : Add +11 to the Given Year. If Result is a leap year, Go to step 2.

Step 2:  Add +6 to the Given Year.

Solution :

Given Year is 2024, Which is a leap year.

So, Add +28 to the given year (i.e 2024 + 28) = 2052

Therfore, The calendar of the year 2024 can be used again in the year 2052.

Repetition of leap year ===> Add +28 to the Given Year.

Repetition of non leap year

Step 1 : Add +11 to the Given Year. If Result is a leap year, Go to step 2.

Step 2:  Add +6 to the Given Year.

Solution :

Given Year is 1993, Which is a non leap year.

Step 1 : Add +11 to the given year (i.e 1993 + 11) = 2004, Which is a leap year.

Step 2 : Add +6 to the given year (i.e 1993 + 6) = 1999

Therfore, The calendar for the year 1993 will be same for the year 1999

First,we count the number of odd days for the left over days in the given period.Here,given period is 18.3.1994 to 25.2.1995
Mar Apr May June July Aug Sept Oct Nov Dec Jan
13 30 31 30 31 31 30 31 30 31 25 (left days)
6 + 2 + 3 + 2 + 3 + 3 + 2 + 3 + 2 + 3 + 4(odd days) = 5 odd day
So,given day Friday + 5 = wednesday is the required result.

Here, after 8 complete weeks again we will get Monday and 5 more days. So, Monday+5=

Saturday

Since within month ( only date different month and year same), we must count the total day after 2, that is from 3 January to 22 January and then divide by 7. Take remainder as ODD /Extra day. Then add an extra day with the given day. That is, 22-2=20/7, and it will go 2 times and the remainder will be 6. Hence given day +6= Wednesday+6= Monday.

Total extra days=12 ( Which is greater than 7, so again must divide by 7 to get a total extra day)

First, write down DBY then Y, TD, T, DAT, 1, 2, 3  serially and then count from Thursday to 3.  3 is Friday. Hence the third day after the day after tomorrow is Friday

First of all, we have to find the day on 01.03.2005

01.03.2005 = (2004 years + Period from 01.01.2005 to 01.03.2005)

Number of odd days in 2000 years = 0 —-(1)
Number of odd days in 4 years = 1 leap year + 3 ordinary year
= 1X2 + 3X1 = 5 days —-(2)

From 01.01.2005 to 01.03.2005, we have,
Jan-31days + Feb-28days + Mar-1day = 60days
= 8weeks + 4days = 4 odd days —-(3)

Adding (1),(2)&(3), we get,total no.of odd days = 0+5+4
= 9 days = 1 week + 2days = 2 odd days.
2 odd days corresponding to Tuesday.So 01.03.2005 was Tuesday. Therefore, Friday lies on 04.03.2005.

Hence, the dates of March,2015 on which Fridays fell are 4,11,18&25