Is it obligatory for a legally responsible person to begin an act of worship knowing that it will be invalidated, such as beginning a fast for someone who will undergo surgery that necessitates breaking it, or for someone who is certain of menstruation?

The validity of acts of worship is conditional upon intention (niyyah (definition)). For the intention to be accepted, it must be resolute and free from hesitation. Therefore, the intention should be based on knowledge, belief, or strong presumption (dhann). If a person intends something about which they are hesitant, then if its realization is preponderant, their intention is valid; otherwise, it is not. The Lawgiver (Allah) considers excuses to be valid when they have actually occurred, not based on the presumption of their future occurrence.