In some European countries, the Christmas gifts are exchanged on the 24th (not the 25th), that is why the countdown ends early.
Yes, the small treats are put into the pockets before the countdown begins, and on each day of advent you search for the relevant number and enjoy the gift inside.
I see the William Morris fabric is still available here.
https://www.quilterstudio.com/Fabrics160.htm.
There are several tutorials on the net how to sew a pocket calendar, I hope this helps.
https://www.angeledenblog.com/2012/1...-tutorial.html
https://www.frugalandthriving.com.au...ndar-tutorial/
https://thediydreamer.com/christmas/...-with-pockets/