Some Muslims in the UK follow moon sightings announced by Saudi Arabia, while others might rely on local sightings.
Weather conditions can affect whether the crescent is seen in Britain, meaning communities can sometimes begin Islamic months on different dates.
Experts say the crescent moon may be difficult to spot with the naked eye in parts of the UK on May 17, but telescopes and clear skies may improve visibility.
Mosques and Islamic organisations across Britain are expected to issue guidance closer to the expected sighting date.
