It seems like a random choice, 4th month of the year and 6 days into it, so why is it the start of the tax year? The start of the tax year was, a long while ago, New Years day.
It was in 1752 that we moved to the Gregorian calendar which meant new year’s day changed from 25th March to 1st January. As part of this, 11 days were dropped to make the calendar fully align with Europe, meaning the day after 2nd Sept was 14th Sept!
This led to complaints that not only were people feeling like they were loosing 11 days of their lives but more that they were expected to pay a full year of tax despite having only 354 days in it. The Treasury decided to keep the tax year as 365 days and the start of the tax year then moved from 25th March to 5th April.
Fast forward another 48 years and the Treasury decided that because under their old calendar system (the Julian calendar) it would have been a leap year but it wasn’t under the Gregorian that the tax year needed to move once again, just the one day this time to the 6th April we know and use now.