Numbers 20:29

They mourned for Aaron thirty days— For such was the usual time of mourning for great persons: they mourned so long for Moses. Deuteronomy 34:8. Hence, probably, arose the custom among the Greeks, which Plutarch mentions: when any one lost an intimate friend, he offered a sacrifice to Apollo; and thirty days after he offered another to Mercury, who was thought to receive the soul of the departed as the earth received his body.