It is customary to change the Parochet, Torah covers, and some other items in the shul to white for Rosh Hashana and Yom Kippur, and it is common practice to keep the white covers up until after Simchat Torah. (The exact time for switching back has been discussed in several other questions.
In the shul in which I grew up, the shuls my grandparents attend, and literally every other shul I have been in at this time of year (including in Israel, and the US mid-Atlantic and Northeast regions), the change to white was always done on Motzei Shabbat before selichot, which this year would have been 8 days before Rosh Hashana. This makes sense to me because coming after midnight to start Selichot is really the first step in the High Holiday season, and seeing the shul ready for Rosh Hashana helps raise awareness of that.
However, in my shul now, we only change to white on Erev Rosh Hashana in the afternoon. I asked the Rabbi, and he said "I've never heard of doing it earlier than that, and this is how we did it in Ner Yisrael."
I looked around in a few sefarim, but I can't find anywhere that explicitly talks about when the switch is done. The Nitei Gavriel says "from Rosh Hashana through after Yom Kippur", which would seem to imply what our Rabbi said, except for the following three observations:
- That doesn't explicitly exclude having done it earlier.
- As discussed in the linked questions, most shuls leave it up far longer than that anyway.
- The Nitei Gavriel's own footnotes say that he only has a source for Yom Kippur, not for Rosh Hashana and the Aseret Yemei Teshuva.
Does anyone know of any sources for the seemingly-quite-common practice of switching the shul to white starting from the beginning of Selichot?