The rules for companies using direct debits state that, if your payment would normally be due to be taken at a weekend (or on a public holiday), they must wait until the next working day before taking it.
I assume that it's so that there's human oversight (rather than having everything done by computers) to deal with problems if things go wrong. i.e. if a company accidentally takes £1000 out of your account, when they should have only taken £10 out, possibly leaving you with no money to buy food with, you'll want to be able to speak to someone at your bank as soon as possible in order to get your money refunded. That could be tricky if, say, the money was taken out on the Saturday of a bank holiday weekend and you'd got to wait until the Tuesday before your bank opened.