IPA: /ˈmʌnθlɔŋ/
KK: /mʌnθlɔŋ/
Something that lasts for a month or about that length of time.
The festival was a monthlong celebration of art and culture.
Monthlong → It is formed from "month" (from Old English *mōnaþ*, meaning a period of time based on the lunar cycle) and "-long" (meaning having a specified duration). The word "monthlong" refers to something that lasts for the duration of a month.
Think of a "month" as a time period and "-long" indicating duration — so monthlong means lasting for a whole month.