If you are paying by cheque, the cheque must reach HM Revenue & Customs (HMRC) by 19thof the month following the end of the tax month or quarter to which it relates. HMRC recommend that you allow at least three working days for the cheque to reach them.
If you are paying electronically, the due date is the 22ndof the month following the end of the tax month or quarter to which it relates.
If the deadline should fall on a weekend or bank holiday, then the due date is the last working day before the normal deadline, i.e. the due date will be earlier than normal.
To remind you, put the dates in your diary, calendar or email account.