The Self Assessment deadline is on 31 January 2016. This is the deadline for the electronic submission of income tax and capital gains tax returns for the year ended 5 April 2015. Missing the deadline by just a day means a £100 fine.
It can take 6-8 days to set you up for electronic filing, so the real deadline is more like 23 January. You would have to act at very high speed to make the deadline now, and nothing must go wrong. It is time to think about the next deadline on 1 March 2016, when you need to pay all the tax that is due, or face a 5% surcharge.