Charles Ellicott Commentary Genesis 50:3

"And forty days were fulfilled for him; for so are fulfilled the days of embalming: and the Egyptians wept for him three-score and ten days." — Genesis 50:3 (ASV)

Forty days. Herodotus (ii. 86) describes the process of embalming as occupying seventy days. However, he was speaking of what he saw at Thebes, whereas Memphis was the Egyptian capital in Joseph’s time, and we are told that the mummies of Thebes are far more perfectly preserved than those of Memphis. Diodorus (i. 91) agrees very nearly with the periods mentioned here, saying that the embalming took somewhat more than thirty days, and the mourning for a king seventy-two.

The usual period of mourning among the Israelites was thirty days (Numbers 20:29 and Deuteronomy 34:8). Probably, therefore, the forty days spent in the embalming were included in the threescore and ten days, during which the Egyptians mourned for Jacob.