Not really sad, but more contemplative. Taking down our Christmas decorations is more bittersweet for me than sad, I guess... realizing another year is over & that our 3 children are another year older, thinking about how many Christmases DH & I have had together, how the years change us, where we are now & wondering where we'll be next Christmas...
We "undecorate" anywhere from New Year's Day to the 12th Day of Christmas (1/6). Like another PP mentioned, by 1/2, the décor starts to look cluttery & sad to me - like it's "taunting" & reminding me "Christmas is over! It's a new year!". This year, due to some other things, we started taking everything down on 1/6 & finished yesterday.
I just have a few wintery decorations out now - snowmen & snowflakes.