In th'olden days financial transactions were done at the four quarters (it's early in the morning so forgive me - the name of the only one I can remember is Michelmas). Also the year was deemed to end in March and the dates between January and March were sometimes written with two years eg February 14th 1539/40. Anyway, one of the quarters is in March, the 25th I think. This was the date that was regarded as the end of the financial year. However, when we changed from the Julian calendar to the Gregorian calendar and 'lost' 11 days, the financial world decided that they were still going to have 365 days in that year and so to accommodate that the end of the financial year was moved to April 5th, where it has of course stayed!
Some people still adhere to the old traditions - my old boss was always expected to pay his rent on the 4 quarter days.
Blimey - I hope someone comes on here who can explain this better than myself! Any problems in understanding me I'm sure you'll be able to go on google and get a better explanation!