The World Federation One Stop Fiqh

Ruling 2670

If a person vows to fast on a specific day, he must fast on that day. In case he intentionally does not fast on that day, he must not only make it up [i.e. keep a qaḍāʾ fast], but give kaffārah as well. However, he can choose to travel on that day and not fast, and in the event that he is already on a journey, it is not necessary for him to make an intention to stay [for ten or more days] and fast. In case a person does not fast due to travelling or some other legitimate excuse (ʿudhr), such as sickness or menstruation (ḥayḍ), it is necessary for that person to keep a qaḍāʾ fast but there is no kaffārah.