Am Samstag, werde ich auf der einer Geburtstagsparty sein.
In this sentence, why does Geburtstagsparty get both a direct and indirect particle? Is the "der" here supposed to be the masculine nominative case, or the feminine dative case? I imagine it's the latter, as einer is strictly feminine. Also, am I correct in assuming the Geburtstagsparty is in the dative case because the speaker is will be "at" (auf) the party?