Before 1752 the financial year ended on Lady Day - one of the 4 quarter days - which is 25 March.
In September alendar in the UK was brought into line with the one used in the rest of Europe. This meant that Sept 3 - 13 were omitted from the calendar.
But the treasury still wanted a full 365 days worth of taxes for the year, so the financial year ended on 5 April in 1753, and that has continued since then.