。
I've arranged catering for tomorrow's party.
Literal
Tomorrow [possessive-の] party [for-には] catering [object-を] arranged.
手配する ('to make arrangements') is the standard verb for booking logistics — taxis, hotels, catering, suspects (in police contexts too). The composite particle には marks 'tomorrow's party' as the beneficiary/target of the arrangement, with emphasis from は. ケータリング is a katakana loanword.