The question is whether werewolves are known and halachically accepted by the Jewish community, or whether we’re talking about individual werejews who are trying to make their personal challenges compatible with a halachic life.
In the former case, a lot of the issues around Shabbat and holiday observance would be simplified using the principle of mitzvot aseh she’ha’zman grama, which means “positive commandments that must occur at a specific time.” The rabbis exempted women from prayer because it occurs at a specific hour of the day, and that is not compatible with the demands of childcare. However, they are obligated for weekly and monthly commandments like Shabbat and holidays, since that’s flexible enough for them to participate. A rabbinic court could use this precedent to create a similar exemption for werewolves, exempting them from all positive commandments that must occur at a specific monthly (rather than daily) time.
Managing on one’s own, the first priority must be human life - including one’s own. If trying to participate in the holiday would possibly endanger themselves or others, the werewolf would be actively forbidden to do so. In fact, I would argue that the werewolf would be positively obligated to take the necessary measures to prevent harm. If they take all possible precautions, and they still harm someone (or even do damage to property), then they are only responsible as an ones - someone to whom an unforeseeable accident has happened. If they took shoddy or irresponsible precautions, then they’d be responsible as a shogeg - someone who errs through carelessness or poor judgment. Finally, if they knew they would inevitably turn into a werewolf but took no precautions to mitigate harm, then they would be responsible as amezid - someone who is deliberately and fully responsible for the wrong they do. I won’t recap the entirety of Jewish civil and criminal law, but in almost all cases the penalties increase as you move up through the three categories.
(One could also argue that the werewolf is a tinok she’nishba, a child without proper knowledge, when in their wolf form. Such a person is not responsible for their actions, not even to the level of an ones. However, the principle of psik reisha lo yamut - that you are responsible for the illegal consequences of legal acts - would suggest that the ones-shogeg-mezid framework is more appropriate. At the time when you were still human, you had the chance to take actions that affected what would happen when you became a wolf.)
Fortunately, the werewolf does escape some problems with mitzvot just by being a wolf. For example, the thirty-nine categories of melacha, work, that are forbidden on Shabbat are precisely and deliberately categories of human work. A wolf doesn’t weave, for example, or light a fire. Even if they, say, did an action that is isomorphic to melacha, like ripping a piece of cloth, they are not capable of intending the action as work.
On the other hand, there are rabbis who would argue that animals are inherently muktza, inappropriate for use on Shabbat. That stricture would apply not to the werewolf themselves, but rather to other Jews who would not be allowed to touch, move, or benefit from the wolf. This is a minority opinion, though, which is a relief to those of us who own pets! It’s not something that would affect werejews in most communities.
Finally, would the werejew be allowed in synagogue in wolf form? I looked at the responsa on guide dogs in synagogue for a sense of how that might shake out. If the werewolf were not acknowledged by the community, then no, they probably wouldn’t be allowed - random animals are not welcome in the synagogue. If they were acknowledged, then they would probably fall under the guide dog exemption; although we do not ordinarily allow animals in shul, the principles of providing access to prayer for all Jews, and not embarrassing your fellow Jew, override the no-dogs baseline. There are a few communities that insist blind Jews must have a human guide inside the synagogue, but in most cases a werejew in wolf form would be welcomed to pray with the community. However, they could not count toward a minyan, a prayer quorum, as they are not capable of participating in spoken prayer.
