This is the 4177th most frequent Korean word.
주말에
"On the weekend."
The word '주말에' (on the weekend) is used here to indicate the time when the action of meeting a friend will take place.
주말에 친구를 만날 거예요.
I will meet a friend on the weekend.
Here, '주말에' specifies the time frame during which the description of the weather applies.
주말에 날씨가 참 좋았어요.
The weather was really nice on the weekend.
In this sentence, '주말에' indicates the time period during which the act of walking in the park occurred.