Is "Der" needed at all?
Samstag ist für die Jungen
can somebody tell me the reason why "der" is here? what significance does it have in this sentenec?
Without "The", it means every Saturday is her day. In this sentence, her day is only this specific Saturday.
shouldn't it be "your" as in plural your instead of her?
You are confusing Ihr with ihr. Auf Deutsch, Großschreibung ist wichtig.
See this declension table in the Wiktionary.