When you file a Form 11 in 2020, it is for income received in 2019 and the tax due on this. You will pay the balance of any tax due plus a preliminary income tax payment for income tax due on your income in 2020.
So the gains made on a disposal in May 2020 are reported on your 2020 tax submission which is due by October 31st 2021.
However, you have to play preliminary income tax for 2020 by mid November - your payment can be based on your income tax due on your 2019 income or your estimate of what will be due on your 2020 income. Remember, when you pay your preliminary income tax in 2020, the year is not yet finished so your income tax due can not be known with any certainty.