Missed obligatory fasts (qaḍāʾ fasts) may be observed on either shorter or longer days, and by doing so a person is discharged from the legal obligation. This is because the make-up of fasting may be performed at any time, except on days when fasting is prohibited. The Messenger of Allah (peace and blessings be upon him) stated that fasting is not permitted on two occasions: the first day of ʿĪd al-Fiṭr and the days of ʿĪd al-Aḍḥā.
(al-Bukhārī, Ṣawm, 66–67)
However, since a person does not know when death will occur, it is recommended that make-up fasts be observed as soon as possible and that one should not deliberately wait for shorter days. This is because intentionally choosing only short days for make-up fasts may suggest an attempt to avoid the burden and hardship of worship.