Some of you may ask: Why Does Ramadan Start on a Different Date?
If you mean: why does Ramadan start on different dates in different countries, then the reason for this goes back to sighting the crescent moon in every country on the 30th night of Sha‘ban (the month before Ramadan) or failure to sight it and then completing Sha‘ban 30 days. So, the crescent is might sometimes be seen in one country which marks the start of Ramadan and sometimes it might not be seen in another country, in which case Ramadan is delayed by a day. Also, the night of 30 Sha‘ban in which the sighting of crescent is pursued might also differ from one country to another, which can also result in a different starting date for Ramadan.
On the other hand, if you mean by “a different date” the fact that Ramadan is not fixed and constant throughout the year like the months of the Gregorian year. In fact, Ramadan passes through al four seasons every 33 lunar years completing a full cycle. During this time, the fasting person has a share of every season and every weather condition. The reason for this difference is that the Islamic Hijri year is a lunar whose months are between 29 and 30 days, not more and not less. As such, the Hijri year advances by 11 days each year, compared to the solar calendar, and Allah knows best.
Source: General Authority of Islamic Affairs & Endowments (GAIAE), UAE
Also, read: Is it permissible to give gift boxes in Ramadan?