One addition to the answer above with respect to one's position for considering an occasion "hayırlı" for himself or not:
One may think that the occasion is "very bad" for himself at the time of happening but he can still consider it as "hayırlı" because in the coming months, he may see that if it happened the other way, it would have been worse for him.
When you say "hayırlı akşamlar" to someone, you are wishing for the best, wishing that whatever the evening brings, either good or bad, it ends up for the person's welfare.
This can look a bit complicated, better to consider it as a kind of philosophy ;)