Sighting of the Moon
it is fard kifayah (a collective duty) on the muslims to make an effort to sight the moon of ramadan on the 29th of sha'ban. it is wrong to depend solely on the calendar and other astronomical calculations to begin the fast, as the prophet (peace be upon him) himself instructed, "begin the fast on the sighting of the moon and break the fast likewise, but if the sky is cloudy (on the 29th of sha'ban), then estimate it (by completing) 30 days of sha'ban.
 collected by al-bukhari (sahih al-bukhari (arabic-english), vol.3, p.69, no.124) and muslim (sahih muslim (english trans.), vol.2, p.524-5, no.2363)