My vets do them on a certain day of the week, so if you put it in the day after, it would have to wait until the week after - although as I said before, the rescues vet dont charge, but you might be waiting a while, I asked 3 times about mine.
Apparently it is cos of the time it can take to fill them in. The rescue's vet dont, but it can take her weeks to fill them in, so would rather pay her to fill them in and get them done the same week.
How do they justify charging you just for filling in a form? Does anyone elses' vets do this?
The only word of warning I would give is check your vets policy on dealing with claims - my vets charge £3.85 for filling in the form, so I have always waited till the end to avoid paying 2 lots of charges.