If you need to register for self-assessment, then there is a certain date that you will need to register by. If you need to file a tax return for the tax year just finished, then you will need to register with HMRC for self-assessments.
If you are self-employed or need to file a tax return for the 2015/16 tax year, then you will need to register by the 5th of October 2016. If you are late to register then you will not need to worry, you will not get a penalty as long as you pay your self-assessment bill in full by the deadline.
It can take 20 working days to complete the registration process.