Fasting: Intention for an Unassigned Obligatory Fast
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1544
There is no problem if someone intentionally does not make the intention to fast until near ẓuhr for an obligatory fast that has not been assigned for a particular day, such as a fast for kaffārah. If a person decides not to fast or is indecisive as to whether he should fast or not, in the event that he has not done anything that invalidates a fast, he can make the intention before ẓuhr to fast and his fast will be valid.