Ramadan dates change every year because, for religious matters, Muslims follow a lunar calendar — that is, one based on the phases of the moon — whose 12 months add up to approximately 354 days. That’s 11 days shorter than the 365 days of the standard Gregorian calendar. Therefore, the Islamic lunar calendar moves backward approximately 11 days each year in relation to the regular Gregorian calendar.
So that means that the first day of the month of Ramadan, which is the ninth month of the Islamic lunar calendar, moves backward by about 11 days each year.
This has a large impact on how people experience Ramadan from year to year. When Ramadan falls in the winter, it’s much easier to fast: the days are shorter, which means you don’t have to fast as long, and it’s colder out, so not being able to drink water all day isn’t as big of a deal, because you’re not sweating as much.