Eclipses cluster into seasons about 173 days apart, which is short of half a year, so they creep backwards through the calendar.

An eclipse needs the Moon near one of the two points where its tilted orbit crosses Earth's. Those points drift slowly backwards, so the Sun reaches them a little sooner each time — every 173.3 days rather than every 182.6. Over nine years an eclipse season works its way all the way round the calendar.