Siman 304

The Rambam wrote at the end of the tenth chapter: One who wears shaatnez or covers himself with it is lashed. If he was wearing shaatnez the entire day, he is only liable for one. If he removed his head from the garment and returned it, removed his head and returned it, even though he did not strip off the entire garment, he is liable for each and every time. In what case is it said that he is liable for one the entire day? When they warned him with a single warning. But if they warned him and said to him, "strip it off, strip it off," and he is wearing it and lingered long enough to strip it off and put it back on after they warned him, he is liable for every lingering for which they warned him, even though he did not strip it off. Thus far his words.

And one should not ask: how can he be liable for each and every duration of stay, for is there not only one act here? For it can be said that shaatnez is not similar to prohibitions of eating where the prohibited item is lost from the world, and every moment that he does not remove the shaatnez from his body, it is as if he put it on; therefore, he is liable for every warning. However, that which he wrote, that if he took out his head and returned it, took it out and returned it, he is liable for each and every one—this is also when there was a warning for every removal and return, for if not so, it is impossible to be lashed twice for one warning (Keseph Mishneh). But according to this, the language is not clear; why did he divide them into two? He should have said: if they warned him and said to him "strip," and he stripped and dressed, or stayed for the duration of stripping and dressing, he is liable for each and every one. However, the law is certainly so, like one who eats several olive-bulks of chelev with large interruptions, and nevertheless is only liable for one set of lashes with one warning. And so it is explained in Makkot (21b) according to Rav Ashi there, that stripping and staying have one law. See there.

He further wrote: One who dresses his fellow in shaatnez—if the wearer was intentional, the wearer is lashed, and the dresser violates "and before a blind person etc." And if the wearer did not know that the garment was shaatnez and the dresser was intentional, the dresser is lashed and the wearer is exempt. Thus far his words. And it is from the Tosefta of Makkot. But the Rishonim have already rejected his words, for it is impossible that the dresser should be lashed (see Kesef Mishneh). And the Rambam follows his own reasoning at the end of the tenth chapter of Nedarim regarding one who vows against his wife and provides for her, that he is lashed; and there too they rejected his words. And we have explained his taste and reasoning, with the help of Heaven, above in siman 220, section 38; see there. And consequently, here too his words are straightforward, for since the dresser performed an action, he is lashed when the wearer is unintentional. But when the wearer is intentional, it is impossible for two to be lashed for one prohibition, except regarding shaving where the verse made the shaver like the shaved, as I wrote at the beginning of siman 181. And the wearer, when he is intentional, is more the master of the action than the dresser; therefore he is lashed and not the dresser. And see what I wrote in siman 369.

He further wrote: Kohanim who wore the priestly garments not during the time of the service, even in the Mikdash, are lashed because of the belt which is shaatnez, and they were only permitted in it during the time of the service, which is a positive commandment like tzitzit. Thus far his words. And his intention is regarding an ordinary Kohen, for as for the Kohen Gadol, there is shaatnez also in the breastplate and the ephod. The Ra'avad contested him, for it is explicitly explained in Yoma (69a) that in the Mikdash it is permitted even not during the time of the service. However, in truth, the Rambam follows his own reasoning in chapter 5 of the Vessels of the Mikdash, law 11, that the belt was not permitted not during the time of the service, see there. (And I wonder at the Kesef Mishneh who did not cite his words in the Vessels of the Mikdash and answered this from his own reasoning, see there). And like the words of the Rambam, it is explicitly stated at the beginning of Arachin, see there.

The Tur and the Shulchan Aruch wrote: It is permitted to make shaatnez garments and to keep them, and they are only prohibited through wearing. Thus far their words. This is a Mishnah at the beginning of the ninth chapter of Kilayim. Seemingly, there is no novelty in this, for the Torah only prohibited wearing and draping. It appears to me that the novelty is that it is permitted even by rabbinic decree, and we do not decree a prohibition lest one wear it (and from this I have a difficulty with the position of the Ran and the Gra above in siman 301, section 5; see there and delve into it).