The dates of Passover (Pesach) and Holy Thursday (Maundy Thursday), are both related to the arrival of spring and the phases of the moon, connecting their celebrants with humanity’s remote ancestors, to whom knowledge of the seasons and the heavenly bodies was not merely interesting but vital for survival. Passover begins on the 14th day of the Hebrew calendar month Nisan, which is also the date of the full moon following the vernal equinox; Holy Thursday falls on the Thursday before Easter, which is the first Sunday following the first full moon after the spring equinox. (Whew!) It sometimes works out that they fall, very satisfactorily (to me, anyway), on the same day; Holy Thursday is, after all, the celebration of a Passover meal.
For a painting and a poem about Holy Thursday, please see Holy Thursday.