The main UK tax dates people actually miss — especially around Self Assessment, tax-year end, and payments on account.
Last day to use many annual allowances for the 2025/26 tax year.
Fresh annual allowances and thresholds kick in for the new tax year.
If you need to file for the first time, this is the usual registration deadline after the tax year ends.
Paper Self Assessment returns generally need to be filed by the end of October.
Usually the big one: online return due and balancing payment due at the same time.
Many self-employed people and landlords pay in two instalments toward the next year's bill.