This question is assuming that one may not tighten a regular tzitzis knot on shabbos, using the rule of “אין עשה דוחה את לא תעשה ועשה” (as seen in Beitza 8b and Bava Metzia 30a), and that without at least one knot on each corner, the tzitzis is passul. If one is wearing a pair of sha’atnez tzitzis on shabbos, (which is muttar mede’oreisa, see ערוך השולחן אורח חיים ט׳ י״ג) and he sees that one of his string’s last knot is loose, with a good chance that it will unravel completely during shabbos. Lede’oraisaay could he tighten the knot, (since this is a case of preventing a bittul of an עשה and לא תעשה- the four cornered garment without tzitzis and the sha’atnez which comes into effect once the mitzva is mevutal) or does he have to wait for it to actually unravel before fixing it, or does he have to take it off completely?

  • Is tying Tzitzit a mitzvat aseh? I've never heard of it. – Double AA May 5 '19 at 11:42
  • How is Shaatnez pushing off Shabbat? Seems like you want Tzitzit to push off two Lavs and an Aseh. – Double AA May 5 '19 at 11:50
  • @Double AA I don’t mean to say that tying tzitzis is a mitzva, rather that wearing kosher tzitzis is, which the garment won’t be if the knot isn’t tied. Why can’t sha’atnez push of shabbos? Stopping himself from wearing Sha’atnez pushes off the lo ta’asei and continuing to wear tzitzis pushes of the asei of shabbos. What’s wrong with that? If you can bring proof and not conjecture that there is something wrong with it, that will be my answer. – Lo ani May 5 '19 at 13:58
  • "Stopping himself from wearing Sha’atnez" The only way to stop himself from wearing Shaatnez is to take it off. Tying Tzitzit would just make the problematic Shaatnez-wearing worth it. You now want to make tying Tzitzit worth it at the cost of both wearing Shaatnez and breaking Shabbat. I don't see how that could be worth it, given the source in your question. – Double AA May 5 '19 at 14:23
  • @Double AA do you know that as a fact? Because if so, then that answers my question. – Lo ani May 5 '19 at 21:05

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