When is the tax year?
In the UK, people get taxed on what they earn from 6 April to the following 5 April.
So for example, the 2020/21 tax year runs from 6 April 2020 to 5 April 2021.
If you are employed and not a very high earner, you don't need to do anything beyond that as your employer would have taken off your tax.
If you are self employed, you will need to complete a tax filing, called a tax return (don't confuse the word return with refund though - most people have to pay!).
The filing needs to be completed by 31 January after the tax year ends. So for the year ended 5 April 2020 the filing was due by 31 January 2021. If you have earned more than the personal allowance (which is around £12,500 depending on the year), you will need to pay tax as well. Some people put money aside as they earn.
Normally, if you submit after the deadline you'd be liable to a £100 penalty. However, this year (2021) HMRC have announced that no-one will be charged a late filing penalty if they file online by 28 February 2021. We are still recommending that people complete a tax return as soon as possible. The quickest and easiest way to do this is with untied.