If someone frowns when you wish them a “Happy Monday,” it might not be the “happy” part to which they object.

The English noun Monday is rooted in an old Germanic term, which in turn is an interpretation of the pagan, pre-Christian Latin lunae dies (“day of the moon”).

“Jewish and Christian traditions place Sunday as the first day of the week, and Monday is thus the second day of the week. Quakers traditionally refer to Monday as “Second Day,” eschewing the pagan origin of the English name “Monday.” For similar reasons the official liturgical calendar of the Roman Catholic Church refers to Monday as the second celebration day – Feria secunda. The Portuguese and the Greek (Eastern Orthodox Church) name for Monday reflects this, as do all the days’ names except Saturday and Sunday: the Portuguese word for Monday is segunda-feira and the Greek word is Δευτέρα “devtéra” (second in order). Likewise the Hebrew name for Monday is yom-sheni (יום שני) (“second day”).” https://en.wikipedia.org/wiki/Monday