Complex sentence: From the sixteenth century it has become customary to number the days from the beginning of the month and February 29th is regarded as the leap day, this being the day which does not occur in three years out of four.
Split into simpler sentences: From the sixteenth century it has become customary to number the days from the beginning of the month. The leap day is regarded as the 29th, this being the day which does not occur in three years out of four.