The deadline for registering for Self Assessment is 5 October following the end of the tax year,

You must tell HMRC that you need to prepare a tax return if you have any of the following:

• Self employed income (sole trader, freelancer, partner etc)
• Made a capital gain.
• Have employment income not taxed under PAYE.
• Have received any dividend income and further tax to pay on that income.
• Any other untaxed income or are liable to tax at higher rates.
• Are due to repay any child benefit

You can register with HMRC for self-assessment here.