The phrase "passive interval" refers to a duration of time characterized by inactivity, lethargy, or a lack of engagement. Some possible antonyms for this term might include "active period," "energetic span," or "engaged interval." Alternatively, one might also use antonyms that convey a sense of urgency or dynamism, such as "critical window," "vital juncture," or "crucial phase." In any case, the antonyms would indicate that the time period in question is marked by energy, action, or the need for swift decision-making, as opposed to a passive or uninvolved approach.