The UK tax year ends on 5 April and the next one begins on 6 April, which is why none of the Self Assessment dates line up with a calendar year. Everything else follows from that boundary: which year an invoice belongs to, when registration is due, when the return has to be in, and when each payment lands. This page sets out the tax return dates in the order they arrive and explains what happens at each one, so a date near the boundary can be placed without guessing.
5 April, and which side of it a payment falls
The year runs 6 April to 5 April. Income is normally allocated to the year in which it arose rather than the year the money was received, so work done on 2 April belongs to the year just ending even if the invoice is paid in May. Expenses follow the same logic. Most disputes about which year something belongs to are really about this distinction, and getting it wrong shifts profit between two returns rather than removing it, which is why HMRC cares about consistency as much as about the individual entry.
5 October: registration
If this is the first year you have income to report, registration for Self Assessment is due by 5 October following the end of that tax year. This is the date people miss, because there is nothing to prompt it and the return itself is not due for another four months. Missing it does not remove the obligation to file, and the later deadlines do not move.
31 October and 31 January: filing
A paper return is due by 31 October following the end of the tax year; an online return by 31 January. The online date is also when the balancing payment for that year falls due. A return filed after its deadline attracts a £100 fixed penalty whether or not any tax is owed, which is the single most avoidable cost in the whole process.
31 January and 31 July: the payments on account
Where payments on account apply, the first falls on 31 January with the balancing payment and the second on 31 July. This is why a first January bill is so often larger than the tax on the year's profit: it is last year's balance plus the first instalment of this year's. The payment sheet on this site separates the two for a given tax year.
Questions people ask about when does the tax year end
Why does the tax year end on 5 April?
History rather than logic: it descends from the old quarter days and the calendar change of 1752, and it has never been moved. What matters in practice is only that income and expenses are allocated to the year ending 5 April.
An invoice was raised in March and paid in May. Which year?
Normally the year the work was done and the income arose, so the year ending 5 April. The date the money landed does not usually decide it.
What if a deadline falls on a weekend?
The published dates are the dates. Do not rely on a weekend giving you extra time; file and pay ahead of the date rather than on it.
Are the dates the same every year?
Yes for the tax year boundary and the filing and payment deadlines. What changes each year are the rates, thresholds and allowances applied inside them.