Affective forecasting, or hedonic forecasting, is the attempt to predict one's emotional state (or affect) in the future. Unfortunately, this is very difficult to do because of the vagaries, or unplanned or unexpected happenings, of daily life.
For instance, an individual is eagerly looking forward to an outdoor birthday party and expecting good weather and sunshine for the occasion. Their mood is upbeat and they expect that mood to continue. This is an example of "affective forecasting."'
However, the weather doesn't cooperate and it storms heavily and the party has to be cancelled. This results in a major mood change; disappointment, unhappiness, etc. This outcome proves the affective forecasting wrong.