The best time is beginning of the year. Beginning of the year for taxpayers in the UK is 6 April 2021.
If your tax return is done early, then you will have the longest time to plan how you will pay your tax bill. Due date for the payments will be 31 January 2022.
Having your Self Assessment done early also means you can get to use it when it is still considered valid or a reasonable basis for earnings. This is in cases of moving home, mortgages, loans etc.