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.