在周末是on weekend还是on the weekend

　　This is to announce the ball on the weekend.

　　兹订于本周末举行舞会。

　　They immediately became friends and start to play at each other's houses on the weekend.

　　他们很快就成了朋友,周末开始在对方家里玩耍。

　　She inflicted her nephew on them for the weekend.

　　她把侄儿打发到他们那儿去度周末，真是添乱。