Question:
Can one do Teshuva on Shabbos/yomtov? Regreting sins and saying ASHOMNU, ect. Or only in ones mind?
Answer:
There is no problem to repent on Shabbos or Yom Tov. On the contrary it is is commendable to repent daily. It is only forbidden to ask a private request that might cause undue anguish.
Sources:
O:CH 188:6 Mishnah Berurah 22
