This is the 481st most frequent Dutch word.
sommige
"Some" or "certain."
In this sentence, 'sommige' is used as a determiner to indicate a non-specific subset of a group, here referring to 'some people'.
Sommige mensen houden van katten.
Some people like cats.
Here, 'sommige' specifies a subset of all possible books, implying 'some books' that the speaker has read.
Ik heb sommige boeken al gelezen.
I have already read some books.
In this context, 'sommige' refers to a subset of days, highlighting their distinguishment as being 'some days'.