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About wearing tzitzis at night. There is a ptur on tzitzis at night, see the sugya in shabbos. Some chassidim wear tzitzis at night though, based on kabbalah.
It’s actually a machlokes, Rashi and Tosafos hold that “day clothing” requires tzitzis even at night. The Rambam holds that there is a general p’tur at night. The Mechaber brings both dei’os.
While we’re on the subject, I’ve been bothered for a while by what I think is a shtarke rayah against the Rambam, so I’ll put it out there and hear what the oilam has to say.
In Shabbos 25: the Gemara tells the story of R’ Yehuda’s Friday afternoon. He would bathe, dress up in linen shawls that had tzitzis of wool on them, and wait for Shabbos to come. The gemara relates how his talmidim used to tuck in the corners of their ????? when they were around him, because they held that mi’drabanan one is not permitted to wear tzitzis of sha’atnez because he may come to do this with a ???? ???? which would be a violation of the issur sha’atnez since there is no mitzva of tzitzis by a ???? ????. R’ Yehuda disagreed about this chashash and therefore he flaunted his woolen tzitzis on his linen shawls.
My question is: Everyone agrees that a ???? ???? of sha’atnez is assur. The machlokes was whether or not we are gozer ??? ???? ????. I would think it quite reasonable to assume that if R’ Yehuda was dressing up for Shabbos he didn’t throw off his clothes as soon as Shabbos came. If so, according to the Rambam that even “day-clothing” is patur from tzitzis at night, how was he allowed to wear these ?????? ????????? without being oiver on the issur sha’atnez?!
(Bear in mind that the gemara in Brachos 20a doesn’t consider sha’atnez shev v’al ta’aseh even though it’s already on the person, also bear in mind the Rambam’s shita that there is no such thing as tosafos shabbos.)