Should they be taken down before February 1 or February 2?

In other words, are Christmas decorations to be up on Candlemass day?

Tomorrow (Saturday) night.

🆗, this year, but what about when Septuagesima starts after Feb. 2?

I'm pretty sure you would go with the February 2 thing.

Are Christmas decorations to be up on February 2 or taken down before February 2 (in the years when septuagesima begins after Feb. 2)?

That is only partially true. As every year, including this year, where Christmas and pre-Lent overlap, we will pray the Preface of Christmas on February 2nd, already in the pre-Lent period.

The Preface of Christmas on Feb 2nd is one thing.

But for those serious Catholics who want to "live the Liturgy" and follow the Liturgical Year in their daily lives, the Church is clear what we should be thinking about the next few days. The Alleluia has been ceremoniously put down, and She has donned the purple vestments of penance.

It actually starts at sundown on the day before.

So we followed the Church's lead and took down all our Christmas decorations today, and sealed away the Christmas music until this December.

You can do whatever you want as head of your own family. Listen to Christmas music during Lent for all I care.

In the Church's liturgical calendar, there are 2 cycles, namely the Temporal Cycle (partially fixed based on Advent-Christmas and movable based on Easter) and the Sanctoral Cycle (fixed by date). Usually Candlemas occurs before Septuagesima. However, this year is a bit peculiar possibly due to the shorter Advent period, thus Septuagesima Sunday falls earlier than Candlemas. Now, since Septuagesima uses liturgical violet to denote penance, it is proper to remove the crib and decorations earlier this year.