This is the 4077th most frequent Turkish word.
haftanın
"Of the week"
Here, 'haftanın' is used as a possessive genitive, meaning 'of the week', specifying the ownership of 'ilk günü' (the first day).
Haftanın ilk günü pazartesidir.
The first day of the week is Monday.
In this sentence, 'haftanın' again means 'of the week' and serves as a qualifier for 'en önemli toplantısı' (most important meeting).
Bu, haftanın en önemli toplantısı olacak.
This will be the most important meeting of the week.
Here, 'haftanın' ('of the week') describes 'başında' (start of), detailing the temporal location.