VAT is paid on the same date the return is due: usually one calendar month and 7 days after the end of the accounting period. There is no separate payment window and no grace after filing. The two questions people actually mean by this are different, though: when each quarter's money is due, and when the obligation to charge VAT at all began. The second is the effective date of registration.
Each quarter's money
The deadline for submitting online is also the deadline for paying, and the payment has to have reached HMRC's account by then rather than merely have been sent. That distinction is the whole practical content of the rule: allow for how your payment method clears. It applies even when the date falls on a weekend or bank holiday. Your VAT online account shows both the return due date and the date the payment must clear, and those are the dates to work from rather than a generic calendar.
When you start paying at all
The obligation runs from your effective date of registration, which is not the date you applied and often not the date the number arrived. Cross the threshold on the backward test and it is the first day of the second month after you went over. Register on the forward test and it is the date you realised turnover would exceed the threshold. You must pay HMRC any VAT you owe from the date they register you, including on invoices raised before the number came through.
Paying in instalments and paying early
Larger businesses make payments on account through the quarter rather than one payment at the end, and businesses on the annual accounting scheme pay instalments through the year with a balancing payment at the end. Everyone else pays once per quarter. If you cannot pay by the deadline, contact HMRC as soon as possible rather than after: late payment interest runs from the first day the payment is overdue until it is paid in full, including when you are paying in instalments.
Questions people ask about vat payment dates
When do I have to pay VAT?
Usually one calendar month and 7 days after the end of the accounting period, the same deadline as the return. The money has to have reached HMRC by then.
When do you start paying VAT?
From your effective date of registration. On the backward threshold test that is the first day of the second month after you went over; on the forward test it is the date you realised you would.
What if I cannot pay on time?
Contact HMRC as soon as possible. Late payment interest runs from the first day the payment is overdue until it is paid in full, and a late payment penalty can apply on top.