In the United Kingdom the tax year traditionally ends on the 5th of April each year. But why is this?
In ancient times the Romans regarded April as the first month of the year. Around 700BC they added 3 months to the year, and January became the first month - but the tradition of counting finances from the start of April had been established.
In the Christian era, the custom became to account for the year that ended on Lady's Day, the 25th of March. This carried on throughout the Middle Ages.
In 1752 the UK moved from the Julian to the Gregorian Calendar - this meant the calendar jumped forward by 11 days that year. The tax authorities didn't want to loose 11 days' revenue, so they arranged for the tax year to end on 5th of April. And it remains so to this day.