Of course I feel that the true meaning of the phrase is when one looks back to better times. Like the halcyon days of youth, running through a summer meadow with the wind in your hair, on the way to mummy and daddy for a lovely picnic, with lashings of ginger beer and scones. Happy Days.
Course, I lived on a rough as fkuk estate, and used to get the shite kicked out of me by my older brother on an almost daily basis. The nearest I got to a summer meadow was when the council forgot to mow the kids park, and the grass got long enough that you just didn't see the dog shite until you landed on it, CNUTS.
"Happy days" can be used in a number of different situations.
1 when you are of good cheer and temperament and nobody has done anything to piss you off lately. (See k19's comment above.)
2 When you have just got away with something illegal, immoral or unethical and no one has sussed you out, that too can be refered to as "Happy days".
3 Something happens to improve your lot in life, such as a lottery win, bonus payment or divorce, that again will be "Happy days"
4 When something unfortunate, but amusing happens to someone that you dont like, such as hurricane in France, Gormless Brown being booted out of office or your ex-wife being run over, that again is "happy days".