I have an extension piece that I can add to my table when we have a lot of guests. If I’m only having guests during Shabbos day, can I add the extension piece to my table on Shabbos day or must I add it before Shabbos? To note, that I always take out the extension piece when I’m done cleaning the table after the meal, and it is not generally left with the table.
Being that the extension piece does not last at all, you may therefore add the extension piece even on Shabbos itself and do not have to do so before Shabbos.
Explanation: It is permitted on Shabbos to attach two pieces together if the two pieces are not meant to last at all and it is done temporarily. Likewise, if the two pieces are very loosely attached it is permitted even if the connection is meant to last for a while. Accordingly, in the above scenario one may add the extension piece to the table even if it involves a strong connection [which usually it does not] being that is not meant to last at all, as she removes it right after the meal.
Sources: See Piskeiy Teshuvos 315:7; SSH”K 24:23; See regarding attaching two items together that is not meant to last at all: Admur 313:20; Beitza 32; Ketzos Hashulchan 119 footnote 34; See Minchas Yitzchak 4:122; See regarding attaching two items together loosely: Admur 313:21; Rama 313:6