IPA: /ˈwiːk.lɔːŋ/
KK: /ˈwiːkˌlɔŋ/
Something that lasts for a whole week.
We attended a weeklong festival in the city.
Weeklong → It is formed from "week" (from Old English *wice*, meaning a period of seven days) and "long" (from Old English *lang*, meaning having a great extent). The word "weeklong" describes something that lasts for the entire duration of a week.
Think of a 'week' as a period of seven days and 'long' meaning it extends through that time — that's why weeklong means lasting for a whole week.