Is it permissible for someone who has vowed to fast for an entire month to have his brothers or sisters help him by fasting some days on his behalf, and is it permissible for him to fast them discontinuously on Mondays and Thursdays with the intention of fulfilling the vow, knowing that he did not intend assistance or continuity when making the vow?

He who vows to fast for a month must fulfill his vow. If the month is not specified and he did not intend consecutive fasting, he may fast intermittently, but he must complete thirty days of fasting. If he begins fasting on the first day of a lunar month, he must fast until its end, even if it is an incomplete month.

However, if he vowed to fast consecutively, or if he follows the Hanbali school of thought, consecutive fasting is obligatory for him.

Proxy fasting is not permissible on behalf of a living person. As for vowing itself, it is disliked (makrooh).