Yes you can pay your employees earlier than usual over the festive period but, it will be necessary to report the normal payment date on the Full Payment Submission (FPS).
HMRC have issued details of these special arrangements for reporting Christmas payroll which was introduced last year to help universal credit claimants where they are paid earlier than usual to ensure their awards are not reduced or stopped.
If you pay your employees early on 20th December 2019 and their normal pay date is 31st December 2019, the payment would be reported as the 31st December 2019 and the Full Payment Submission (FPS) would be sent on or before the 31st December 2019.
Please see further guidance from HMRC here. Outside of December, normal 'on-or-before' rules apply.