We don't "do" Thanksgiving in Ireland (although, I usually do a mid-week after work version of a roast turkey dinner on the day). But it also means we don't have that long weekend to play with.
If we are staying here, I will get a real tree either at the school fair (2nd weekend of December) or from another local supplier (local sports clubs sell them, and I have a few suppliers who do it as well as their regular business like F&V shop - I don't like to buy from the trailer on the side of the street type operations). We might put it up that weekend or the following weekend, or maybe even mid-week - all depending on how manic we all are. But it's usually well up before the weekend before Christmas.
The years we travel, the fake tree comes out of the attic anytime between 1-15th December, realistically. Again, we tend to do it on a weekend, but some years it has been mid-week. This year, I am actually thinking of the weekend of 7th December - as DD and I are at home with not too much on (an art project on Saturday afternoon, school fair on Sunday afternoon - and regular weekly sports training both mornings), and it would be a nice surprise when DH gets home the following weekend (when we still have training both mornings, but DD has 2 Santa parties in the afternoon - my office and one of her sports clubs, 1st Communion preparation mass on Sunday (which we HAVE to get to, rather than the regular mass we try to get to before training but can miss), and potentially the Santa steam train trip for us all that afternoon - and is our last weekend in this city before travelling so any last buying that DH or DD want to do needs to get done - I can still grab things midweek around work).
So I think, realistically, having it done before DH comes home is the best option. In fact, we may even do it the afternoon he goes (1st Dec) to distract ourselves and get to enjoy it longer.
Oh, and while our temporary landlords for Christmas have said they will put up a little tree in the cottage, I anticipate bringing a few decorations with us and doing something in our first few days to make it festive and keep DD occupied! (So, between 20th and 24th).