Car tax is due at the end of the month and I'm sure I've normally had the reminder quite early on in the month.
I know it is possible to tax it at the post office with all the documents but is easier to do it online with the reference number in the letter. Am hoping it arrives soon and I can do it (am moving house soon and everything is in various places, not all known to me).
As an aside it would be good if you could pay by direct debit wouldn't it?