Tax for a tax year is paid in two stages. You make two payments on account of income tax for the tax year. The first payment on account is made on 31 January in the relevant tax year and the second is made six months later, on 31 July following the tax year.
For the tax year 2019/20, the first payment on account is made on 31st January 2020.
The second payment on account is made six months later, on 31 July 2020.
You should submit your self-assessment return no later than 31 January 2021 following the tax year and at this point you will be in a position to finalise your tax liability for the tax year – therefore you may need to make a final balancing payment for 2019/20 on this date, if your payments on account fall short.
Don’t forget that on 31st January 2021 you also have to make your first payment on account of your 2020/21 tax liability for the following year.