The UK tax year starts in April because of a chain of calendar accidents: a medieval religious holiday fixed the year-end at March 25, Britain’s 1752 switch from the Julian to the Gregorian calendar wiped out eleven days, and the Treasury shifted the tax year-end forward to April 5 (and later April 6) to avoid losing any revenue in the reshuffle. So when people ask why does the UK tax year start in April rather than January, the short answer is that the fiscal calendar was frozen in place while the civil calendar moved on around it.
Lady Day and the Old Legal Year
For nearly six centuries, England’s legal year did not begin on January 1. It began on March 25, the Feast of the Annunciation, known as Lady Day. From 1155 onward, this religious holiday doubled as the administrative starting gun for government, commerce, and the courts.1Wikipedia. Lady Day Tax assessments, rent collections, and employment contracts all pivoted around it.
Lady Day was one of four “quarter days” that carved the English year into rough blocks. The others were Midsummer Day on June 24, Michaelmas on September 29, and Christmas on December 25. Rents came due on these dates, leases began and ended on them, and servants were hired or dismissed by the quarterly rhythm.2Britannica. Quarter Days Of the four, Lady Day carried the most weight because it separated one legal year from the next. Books were closed, debts settled, and the cycle reset.
Because the legal year and the fiscal year were the same thing, every arm of the state assumed March 25 was the hinge date. Court terms, parliamentary sessions, and revenue tallies all lined up to it. That held for centuries, right up until the calendar itself had to change.
The 1752 Calendar Switch
By the mid-1700s, Britain was still using the Julian calendar, which overestimated the solar year by about eleven minutes. Over centuries, those spare minutes accumulated into a drift of eleven full days. Most of Catholic and Protestant Europe had already adopted the more accurate Gregorian calendar, leaving Britain out of step with its trading partners.
Parliament fixed the drift with the Calendar (New Style) Act 1750, which ordered two changes. The legal year would henceforth begin on January 1 instead of March 25. And eleven days would be stripped from September 1752 to realign with the sun: the day after September 2 became September 14.3Legislation.gov.uk. Calendar (New Style) Act 1750
The civil calendar now matched the rest of Europe. But cutting eleven days out of the year created an immediate problem for anyone counting money. Every contract, lease, and tax obligation written for a 365-day year was suddenly short.
How the Treasury Kept Its Full Year
The Treasury was not about to swallow eleven days of lost tax receipts. If the fiscal year still ended on March 25, the government would collect revenue for only 354 days instead of 365. That gap would have punched a hole in the national budget at a time when military and colonial spending demanded every penny.
The fix was simple. Keep the tax year at a full 365 days by moving the end date forward. Officials added the eleven missing days to March 25 and landed on April 5. From that point on, the tax year ran from April 6 to April 5, even though the civil and legal year now started on January 1.4Chartered Institute of Taxation. Time to Ditch Quirky 5 April Tax Year End The fiscal calendar and the civil calendar, previously identical, were now permanently split.
No taxpayer could argue they owed less because Parliament had reshuffled the months. No landlord could claim a shorter lease. The full annual cycle of obligations survived intact, just anchored to a different date.
Why April 6 and Not April 5
The date shifted one last time at the turn of the nineteenth century, thanks to a quirk in how the two calendar systems handled leap years. Under the Julian rules, any year divisible by four was a leap year, which made 1800 a leap year. The Gregorian calendar added an extra condition: century years count as leap years only if they are divisible by 400. So 1800 was not a leap year under the Gregorian system.5U.S. Naval Observatory. Leap Years
The Treasury, consistent as ever, chose the interpretation that preserved revenue. Treating 1800 as a leap year for tax purposes gave the fiscal year one extra day, and the start of the tax year moved from April 5 to April 6, where it has stayed.4Chartered Institute of Taxation. Time to Ditch Quirky 5 April Tax Year End
The same logic could have triggered another shift in 1900, which was also a Julian leap year but not a Gregorian one. By then, the tax authorities simply stopped adjusting. April 6 was fixed, and nobody moved it again.
Two Tax Calendars, Not One
One detail worth knowing: the UK actually runs two fiscal calendars in parallel. The personal tax year for individuals runs from April 6 to April 5, rooted in the history above. The government’s own financial year, and the one used for corporation tax, runs from April 1 to March 31.6GOV.UK. Review of Potential for Moving the Tax Year End Date: Scoping Document
The five-day gap exists because the government’s financial year was later standardized to cleaner calendar boundaries, while the personal tax year kept its historical quirk. In practice, a sole trader filing personal tax returns follows the April 6 cycle, while the same person’s limited company files corporation tax on the April 1 cycle.
Why the Date Has Never Been Changed
The Office of Tax Simplification reviewed the question in 2021, looking at whether the personal tax year should move to December 31 (matching the calendar year) or March 31 (matching the government’s financial year). The review acknowledged that a calendar-year alignment would be “the natural, simplest and easiest approach for everyone to understand,” while a March 31 move would involve less disruption and would close the gap between the personal and corporate tax calendars.7GOV.UK. Exploring a Change to the UK Tax Year End Date
Neither option was formally recommended. The report noted that any shift would need to wait for the completion of major HMRC modernization projects, and that the systems impact on both government and the private sector would be substantial. April 6 remains, sustained less by logic than by the sheer weight of infrastructure built around it over the past 270 years. A medieval feast day, a calendar correction, and a Treasury that refused to lose a single day of revenue: that is why the UK tax year starts in April.