由于我届时已有约会／全部时间已经排满／另有事务性访问，故不能出席。 Because of a previous engagement/full schedule/business trip, I can't make it at the time.
玛丽将在最近某一天完婚，届时你会在哪儿？ Mary will get married one of these fine days, and then where will you be?
由于我们届时已有约会／全部活动已排满／需要出差，故不能出席此次晚餐／午餐，实为抱歉。 I/We regret that I/We can't make it because of a previous engagement/full schedule/business trip at that time.