желаю тебе хорошей пятницы

[Eddie]

желаю тебе хорошей пятницы.
why тебе(dative case)
rather than тебя(accusative case)
Thanks

[Siweida]

Eddie, желаю (кому?) тебе.
Кому? чему? dative case

[Груша ненадолго]

Because one of the meanings of the dative case is to "give" something to someone. This "something" can be either material or abstract. In this situation you are "giving" you "wish" to the other person.