Field day is an expression that typically connotes a sense of enjoyment or excitement associated with an event or occasion. However, there are a number of antonyms for this phrase that could be employed in different contexts to convey a contrasting meaning. Some possible antonyms for field day might include phrases such as bad day, boring day, sad day, difficult day, or unpleasant day. These expressions could be used to describe events that are characterized by frustration, disappointment, or hardship, rather than the positive and upbeat energy that is often associated with a field day. Ultimately, the choice of antonym will depend on the specific context and intended tone of a given communication.