To determine the date of Easter:
Start from March 21st. Look for the date of the next full moon. (If there's actually a full moon on the 21st, as there was this year, that counts). Easter Day is the next Sunday.
http://www.independent.co.uk/news/uk/this-brit ain/the-big-question-why-do-we-celebrate-easte r-and-where-did-the-bunny-come-from-443550.htm l
Different countries used to use different dates to put the clocks forward and back. That could cause problems with, say, ferry timetables. For example, France would be one hour ahead of the UK for most of the year but there could be a week or two where we were on the same time or where France was 2 hours ahead. Such problems led to international agreements that the clocks go forward on the last Sunday in March and back on the last Sunday in October.
Chris