It could only really be this today.
Which is by Lewitt Him and dates from 1941.
As has been pointed out in the comments, the last posting day on all of these posters has been a very consistent 20th December throughout, even in wartime, which is very British and reassuring.
But what intrigued me is that there is no mention of first or second class post – just different posting days for parcels or cards and letters. So I asked the BPMA, who told me that first and second class was only introduced in 1968. I’d always thought – that like first and third class railway carriages – it was the invention of the Victorians. But now I know – better informed thanks to the internet (and the lovely postal archive people).
Don’t panic though, in this modern age of segregated post, you can still get your first class items off tomorrow.