Ramadan is the clearest demonstration of what a purely lunar calendar does, and the mechanism is worth stating exactly.
The Islamic year is twelve lunar months and comes to 354 or 355 days. The solar year is about 365.24. The difference is roughly eleven days, and because the Islamic calendar contains no intercalation whatever, that difference is never corrected. Ramadan therefore falls about eleven days earlier in the Gregorian calendar each year, and works its way through the entire seasonal cycle in about 33 years.
This is not an oversight. Intercalation, nasi, was practised in pre-Islamic Arabia, and it is prohibited in the Quran at 9:37, which describes it as an increase in unbelief by which those who disbelieve are led astray, making a month lawful one year and forbidden the next. The verse is understood as forbidding the manipulation of the sacred months, and the effect is a calendar that is purely lunar by design.
The practical consequences are considerable and are the reason Muslims frequently mention them. When Ramadan falls in a northern summer, the fast from dawn to sunset can run to eighteen or nineteen hours in Britain or Scandinavia and does not end at all above the Arctic circle in some weeks, for which jurists have issued rulings directing the use of the timings of Mecca or of the nearest moderate latitude. When it falls in winter it may be nine or ten hours. Over a lifetime a Muslim fasts in every season, which is often described within the tradition as an equalising feature rather than an inconvenience.