There's always a debate as to when the moon was sighted and people don't always agree on the same dates. What can you do if your family and friends begin or end Ramadan on a day differently than you?
The Islamic calendar is based on the lunar calendar, and so the start and end dates for Ramadan and Eid differ every year... But we have to maintain a sense of unity.
Shaykh Yasir Qadhi discusses and answers the question