Eid al Fitr, the holiday following Ramadan, is declared when the crescent moon is seen, much like how Jews used to declare the new month in Temple times.
Unlike the Jews of ancient Israel and Judah, there is no centralized Muslim system for who determines the date that the new moon is seen.
So this year the holiday of Eid al Fitr will be today in some countries and tomorrow elsewhere.
In Saudi Arabia, Qatar, Kuwait, Lebanon, Iraq and Yemen the completion of the month of Ramadan for the Muslim year 1440 has been declared over and today is the first day of Eid al-Fitr, since they saw the crescent moon.
However, for Egypt and Jordan (and the Palestinian territories), they did not see the crescent, so Tuesday is the end of Ramadan, and Wednesday, June 5, is the first day of Eid.
Hamas seems to follow the Mufti of Jerusalem on this topic.
Jews created a fixed calendar in order to avoid these types of issues during the first millennium of the common era.