Siman 654

We learned in a Mishnah (42a): A woman may receive [the lulav] from the hand of her son and from the hand of her husband, and return it to the water on Shabbat – during the time when they would recite the blessing over the lulav on Shabbat, as will be explained in siman 658. And the lulav stands in water so that it does not wither, and it is permitted to return it to that same water on Shabbat. And even though repairing a vessel is forbidden on Shabbat, nevertheless, since it stood in this water, it does not appear like repairing a vessel. And even the woman, who is exempt from lulav – we do not say that the lulav is muktzeh for her and she is forbidden to handle it. For it is not so; since it is fit for a man, it is also permitted for the woman (Gemara). But nowadays, when we do not recite the blessing on Shabbat, it is absolute muktzeh and is forbidden to be handled.

And it was only permitted on Shabbat to return it to those same waters, but not to add other water, and certainly not to replace them entirely. And on Yom Tov they permitted adding water, but to replace entirely with other water is forbidden, as it is like fixing a vessel. And on Chol HaMoed, it is a mitzvah to replace with other water entirely, so that it remains moist and beautiful. And it seems to me that the addition on Yom Tov must be less than half, for with more it is not called an "addition." And it is proper to take a new aravah every day of Chol HaMoed and tie it to the lulav, and this is a beautification of the mitzvah. One must only be careful that if he inserts it into the binding, he should not pull it from below with force and detach most of the leaves.