When did the word "Gay" change it's meaning?

Not sure, but maybe in the late 1960s or early 1970s.  It was in the early or mid 1970s when I first heard it.  I was in college.

I was a child in the 1950s and 1960s and gay meant happy, like in Christmas songs. 

Of course, as a child, I really wasn't sexually precocious like children nowadays, so I had no clue about what homosexuality was because I barely understood heterosexuality. 

The first word about homosexuality that I heard and learned the meaning of was "queer".  This was in the 1960s.

The meaning expanded around the time of the gay liberation movement (Stonewall). It still can mean happy, but the context is what provides the correct meaning.

My understanding is that it was used as a sort of secret code in the 1960s when, certainly in Great Britain, homosexuality was illegal until 1967 when it stopped being a crime. The word 'gay' was actually the initials G A Y, which was a sort of code meaning 'Good As You' so if someone said "I feel really gay today." it was a signal to anyone who was also of the same sexual persuasion that this was someone they could interact with sexually. Sadly that destroyed the word's real meaning, which is to be frivolously happy.

My memory goes back to the 1950s. I don't think I heard it used on the media, until the late 1960s. This is when all the cultural change started happening. So this seems when the word went mainstream. 

But posters say it was probably an underground term way back. 

 It became sexualized when referenced to young male prostitutes. They were clients of rich men and were carefree and didn't seem to care about work.  Gertrude Stein did use it in 1922 for a homisexual.

Easy enough to google. Wikipedia says it was probably first done before 1950 because that was the first known documented use. Might go back to the 1920s.

It didn't change. Gay still means happy. You will just have to rely on context to know what it's referring to.

Probably sometime during the Eighties, 'cos I can just about remember, as a kid during the Eighties, that it still meant "happy" and "bright", that that it was also starting to take on a brand-spanking new meaning. Now, as a kid, I didn't know what that new meaning was. All I knew was that in that new meaning, it was a "rude word" and a "grown-up word", that I wasn't allowed to use it in that new context, and that I'd be in trouble if I did, and so, evidently, during the Eighties, it was already starting to make that transition. 

"The term's use as a reference to male homosexuality may date as early as the late 19th century, but its use gradually increased in the mid-20th century.[2]" Wikipedia

Words change their meaning all the time. English is a flexible language. "Bling" didn't use to exist. "Rap" used to mean a knock on a door. "Weed" used to mean an unwanted plant in the garden. 

When the gays stole it to make their sick lifestyle CHOICES seem less harmful than they actually are!