it is omission. よい一日を（お過ごしください）。 お食事を御注文のお客様全員にアイスクリームを（サービスします）。 When we can expected after words,we omission it. and,we can omission after "を". を cannot omission. (it is rule,so i cannot explain why.) よい一日を。 is an idiomatic phrase,so usually, we don't say after を words.