If a service is paid for (or a VAT invoice is raised) in advance, the “tax point” (the important date for calculating VAT) is the earlier of the payment or invoice dates.
I’m assuming though that this is a “stand alone” supply of catering being provided at a wedding organised by someone else, rather than being part of a wedding package. Some years ago, the courts decided that a wedding package was a single service being supplied, rather than separate supplies of the various elements included within the package. This stopped venues treating part of their packages as a VAT exempt “room hire”. So for the same reason, I suspect that venues wouldn’t be able to treat part of a package as a supply of reduced rate catering. I can’t see any specific mention of wedding packages in the legislation for the temporary reduced rate, so I’m assuming that 20% VAT still applies to them.