Why does a date which fell on a Sunday between March and December of two identical odd-numbered years always fall on a Thursday in the leap year halfway between the two occurrences of the Sunday?

Answer 1

There will always be a standard pattern to the way days are laid out. If you take a random period that complies to your example, you will see how the leap year causes the Thursday to occur:

Sunday 18 April 1993

Monday 18 April 1994

Tuesday 18 April 1995

Thursday 18 April 1996 *Leap Year

Friday 18 April 1997

Saturday 18 April 1998

Sunday 18 April 1999

3 years before or after a leap year has to be an odd numbered year. If it is a Thursday in the leap, then 3 years before and after it will be a Sunday. It only applies from March to December for a particular year, because the impact of the 29th of February being an extra day does not affect January or February in a leap year. A date in January or February in a leap year, will not have skipped the extra day from the previous year, like some people think it does. However, dates in the January and February of the year after a leap year will always have skipped an extra day. It is Thursday the 26th of February 2015 at the time the question is being answered. The 26th of February in 2016, which is a leap year, will be on a Friday. The extra day will not be skipped until 2017, when the 26th of February will be a Sunday.