Just a thread on reddit touching about this issue: my guild took a week off during the week of Thanksgiving since everyone was travelling and people wouldn't be able to make it to Mythic raiding progression.
The week after, 14 people logged on. Before, we had trouble getting the full 20 but had a consisent 18-19 logging on.
Now? 7 of my guild mates who were logging on every day before the break haven't bothered to log on.
We're actually progressing decently too as we're tackling Fetid Devourer which we almost have down and will move on to Zul.
Thanksgiving break has been a deathknell for many guilds. It has been for mine. TBH, I'm guilty of it too since I've only logged for raid time last Tuesday and Wednesday and haven't bothered to log back on this week at all.
8.1 can't come fast enough but even then, not sure if it will be enough content until 8.2