Why does the timing of the month of Ramadan change annually, while the timing of Christian holidays is fixed?

The timing of the month of Ramadan (definition) changes according to the Gregorian calendar, just as the holidays of non-Muslims change according to the Hijri calendar. This is due to the Gregorian calendar being linked to the solar calendar and its stability in relation to the seasons of the year, while the Hijri calendar is linked to the lunar calendar. There is a difference in the number of days between the Gregorian and Hijri years, as the Hijri year is shorter than the Gregorian year.