Rosy retrospection refers to the finding that subjects rate events well after they occur more positively than they had rated them immediately after they occur, reminiscent of the Latin phrase memoria praeteritorum bonorum ("The memory of the good pasts").
The effect appears to be stronger with moderately pleasant events and is usually explained as a result of minor annoyances and dislikes "fading" from memory dramatically faster than positive situations.
A related idiom, to "see through rose-tinted (or "rose-colored") glasses", means a perception of a situation or a past event in an over-positive way, not noticing or recalling undesirable or negative aspects.
Experiments
In one group of experiments, three groups going on different vacations were interviewed before, during and after their journeys. Most followed the pattern of initial anticipation, followed by mild disappointment. Generally, most subjects some time later reviewed the events more favorably than they did while experiencing them.