The time for a mustaḥabb fast starts from the beginning of the night but continues to a little while before maghrib adhān so that one has enough time to intend fasting.

It suffices to intend fasting for one month at the night before the fisrt day of Ramadan. However, it is better to intend every night to fast the next day.

'Uṣr and Ḥaraj
If he keeps the fast and breaks it only when it becomes — due to hunger and thirst — unbearably hard for him to continue, he has only to perform qaḍā’ of the fast and no kaffārah is required. Regarding thirsty, he should — by obligatory caution — avoid fast invalidators the rest of the day. 

Fear of Harm
A pregnant woman who has reasonable grounds to fear that fasting would harm her, her fetus, or to her breast-fed baby due to decrease in milk, then, it is obligatory for her not to fast. If she fasts, it is not valid. 

A Physician's Opinion
If one is confident — due to what their physician said — that fasting is harmful for their health or they fear so, then it is not obligatory for them to fast.