Half-Day Fast
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Question
Is it possible to accept a fast for half a day with the condition that if desired, it can be extended to the whole day?
Answer
Hello,
It is possible, but it will not be considered as a fast for a whole day, rather as a 'fast of hours'. Since there was no acceptance from the previous day, a fast without prior acceptance is not a complete fast.
[There are some prayer books that mention a formula to accept a fast for a whole day, adding 'and if I wish to stop and eat, it will not be an obligation', and it seems that this is also not agreed upon, therefore one should accept a fast for a whole day, and if he wishes to stop or changes his mind, he should perform a release of vows].
Source
Shulchan Aruch, Orach Chaim 562:11, and its commentaries.