Self Assessment dates and periods
I'm new to the whole self assessment and tax return process and have few questions. I have a job and get a monthly salary through PAYE, I therefore never needed to file a self assessment. Recently I got into trading and I think I now need to send one. It hasn't been requested by HMRC however it's my take after reading their conditions on capital gains report - you need to report if your capital gains are above the CGT allowance or if total sold assets amount is higher than 4 times the CGT allowance.
So first of all I'm not sure if I should just report it by myself whenever I fall into one of these conditions or wait for HMRC to request it, if someone could confirm this it would be great.
Now the main topic is about dates. I know that the financial year is April to April, however I read that the self assessment and tax return have to be made by end of January and then there is something about 2 payments on account that I don't understand. Let me set up an example to picture it:
April 2020 - start of financial year 2020-2021 - I know that I will earn a stable salary of 50,000£ through the year and I don't have to worry about the self assessment because it is going through PAYE. By November 2020 I make a separate profit with investments or trading and I have now capital gains equal to 20,000£. Am I now required to file a self assessment by January 2021 and pay the due capital gains taxes at the same time ? What if my capital gains go up again in February, since it is still the same financial year when am I supposed to pay taxes for this part ?
And how would these payments on account play out in this same scenario ?