Self Assessment is the process of completing a tax return each tax year. On your Self Assessment you show your personal income and capital gains, and claim tax allowances or reliefs. The Self Assessment return is filed with HMRC and makes them aware of any tax that you are due to pay to them. All self-employed individuals and company directors must file a tax return each tax year.
If you send in a paper tax return it must reach HMRC by midnight on 31 October. If you send in your tax return online it must reach HMRC by midnight on 31 January. You'll have to pay a penalty if HMRC doesn't receive your tax return on time. The later you send your return, the more penalties you're likely to pay.