1. Can a person say Ashomnu every day?
2. A person wanting to do Tshuva. Can it be said on Shabbos as well?
- There is nothing wrong with saying ashamnu everyday. I fact this is the minhag of the Sefardim,to say it twice daily before saying tachanun. However if it isn’t your minhag and you are not saying it during davening, it should only be said if you really mean it. Aside from this, even if you do want to say it, we cannot say the 13 middos ( H-shem, H-shem etc.) unless we are saying it with a minyan.
- It is controversial if one may say viduy on shabbos or not. See M:B 288-26, Sharei Teshuva O:CH 300-1 in the name of the Arizal, who say that it may not be said. On the other hand see Shulcan Aruc Horav 288-8 that says that it may be said, also see Shela Hakadosh beginning of Meseches Roh Hashana D:H Kosuv B’Maharil.