Forever a day is an idiomatic expression used to indicate that something will last for a very long time, often indefinitely or for an eternity. The opposite of forever a day is a fleeting moment, which refers to something that is transitory or brief. Other antonyms for forever a day include temporarily, intermittently, sporadically, impermanently, briefly, occasionally, and semi-permanently. These words are used to suggest that something will only last for a short period, rather than a prolonged or permanent duration. So, when someone says something will last forever a day, the opposite is a brief time.