It is required for its validity to have the intention from the night before, or an intention concurrent with the dawn; this is because this fast is not predetermined, so it is necessary to have the intention beforehand to specify it, as mentioned in Sharh al-Wiqayah p. 234, and Umdat al-Ri'ayah 1: 307. If one fasts with the intention after dawn, it is considered a voluntary fast, and he must make up this voluntary fast if it is invalidated.