A 70's-era American TV series set in the 1950's, and starring, among others, Henry Winkler and Ron Howard. It was a very wholesome, family-oriented show, and was very loved for it's feel-good quality, and overall innocence, and remains quite popular to this day.
Can be anything you want it to be. Such as......
Someone standing in dog shit and angrily looking down at his shit stained kicks "happy days lad😒"
Or a lad who has just bummed a stripper and is now spilling the beans to the lads down the boozer......
"happy days lad 😁"
Was bucking this slut up the shiter and when I pulled my bobbies nut out if her back eye I only had a bit of sweetcorn stuck in me japs eye......happy days lad (angry context)