Fast when you can see the crescent moon and complete 30 days if you cannot physically see it due to weather conditions or other factors .

The hadith narrated by Ibn 'Abbas states that Muslims should fast during Ramadan when they can see the crescent moon. This means that they should start fasting on the first day of Ramadan and continue until they can see it again at the end of a month. If clouds prevent them from seeing it, then they should complete 30 days in order to finish their fasting period for that month. This hadith also teaches us about patience and perseverance as we must wait for a certain amount of time before we can break our fasts even if we cannot physically see it due to weather conditions or other factors. Additionally, this hadith emphasizes on following Islamic teachings even if there are obstacles in our way as completing 30 days will ensure that one has fulfilled their religious obligations for that particular month.