IF u consider the months from March to December(i.e the month after leap year months) then for every year just before leap year ,there is 5 days gap between for any particular date.
Example if 18 april 2009 is Saturday, 18 April 2008 was friday.
But 2008 being a leap year,18 april 2007 must have been wednesday
so if we want to find out 18 april 2005 we follow the following steps:
for every year before a leap year there is a gap of 5 days.
such as 18 april 2007 if wednesday,18 april 2003 wld be friday
so for gap of x leap years there is a gap of 5*x days.
So 18 april 1985,if we consider from 18 april 2007,there is a gap of 5 such years(2003,1999,1995,1991,1987)
so no of days gap is 5*5=25
now 25 days gap from 18 april 2007 will be
now see for 7 days gap,we arrive at same day
so any multiples of 7 will arrive at same day
so if it had been 21 days gap from 18 april 2007 (wednesday),we would have arrived at wednesday,but since it is 25 days gap,see 4 days back calculation from wednesday
so 18 april 1987 was saturday
now 1985 is still two years back
so 18 april 1985 was thursday.
I hope i have made the calculation part clear.
pls do ask me in case of any confusion.
its a very interesting mathematics and very simple to learn.