There are multiple deadlines to remember. Most importantly:
31 October deadline for filing a paper tax return (if you use Earnr, this one doesn't apply as we work with digital tax returns)
31 January deadline for filing online tax return and paying your tax bill. If you use Earnr, we'll remind you when the deadline approaches and help you to file your tax return online.
Other important dates to keep in mind are:
6 April - start of the new tax year
5 October - deadline for registering for Self Assessment