Þriliða is no ordinary month but the inserted thirteenth moon of leap years – „the third Litha". It is the reason the lunar calendar does not drift out of the seasons.
Name & Meaning
Twelve moons are about eleven days shorter than the solar year. So that the months do not slip forward year by year, an extra moon was inserted every few years – by Bede a third Litha in high summer. A year with 13 months the Anglo-Saxons therefore called Þrilíði.
The Balance of Moon and Sun
Over the years this yields a rhythmic leap-cycle (the octaeteris, three leap years in eight). Just this device turns a pure lunar calendar into a lunisolar one: the moon sets the beat, a single solar point – the winter solstice – holds everything in place. The leap month has no feast of its own; it is the quiet mechanism behind the year.
Bede’s Testimony
But when a leap year fell …, the year had three Litha months; and so they called such a year Þrilíði.
— Bede, De temporum ratione ch. 15.
Sources
Bede, De temporum ratione ch. 15; Andreas Zautner, Der gebundene Mondkalender der Germanen (lunisolar reconstruction, octaeteris).