Une lune de miel

Literally: A honey moon

Example: “La lune de miel entre le nouveau maire et l'opposition n'a duré que trois semaines.” — The honeymoon period between the new mayor and the opposition lasted only three weeks.

English equivalent: The honeymoon period

Origin: A calque of English honeymoon, which is attested from the 16th century and may allude to honeyed drink taken for the first month, or simply to sweetness that wanes as the moon does. French took the compound intact in the 19th century.