The 1st day of Christmas is 25th of December and the 12th day of Christmas is 5th of January.
When do we say "Merry Christmas" - A normal protocol is that this is for Christmas Day, so if you will see someone then, wait until the day. If not, then the last time before Christmas. A card is sent for Christmas Day, so no problems there.
Merry or Happy Christmas? - In England you can use either. Merry is more of a "party on" state, whereas Happy is plain "be happy", but used for every other occasions too.
When is Advent? - Normally starting on the 1st December ( if you have an Advent calendar). In the Church calendar there are 4 Sundays before Christmas, so if Christmas Day is on a Sunday, then, as this year, the First Sunday of Advent is on 27th November!
I hope this has been useful. Back in the 40's things would have been very traditional.