For the tax year ending 5 April 2018, the deadline for submitting your Self Assessment online is midnight on 31 January 2019. This is also the date that you must pay any tax which is due. If you submit paper returns (SA100) then the date for submission is earlier; this year the deadline falls at midnight on 31st October 2018.
If you submit your Self Assessment late, you will be fined £100 by HMRC. Should your return be 3 months late you will incur a daily penalty of £10 until it is filed.
You will also be fined if you fail to pay any tax you are liable for by 31 January, and interest will accrue on the owed balance until you bring your account up to date.