Understanding the history and context of the LGBTQ+ pride month that currently exists is really important, especially since the history tends to drastically erase major parts and key players. The current pride month happens in June every year, with major cities often having large parades and festivals during one of the weekends. But it was originally started as a way to commemorate the Stonewall Inn Riots in New York City that took place in June of 1969.
The riot took place in front of the Stonewall Inn in the Greenwich Village of New York City after a police raid had begun arresting patrons of the bar. Raids like this occurred frequently and during the time, were perfectly legal because as the Stonewall Inn website states:
In 1969 Police raids on gay bars occurred regularly. It was illegal to serve Gay people alcohol or for Gays to dance with one another. During a typical raid, the lights were turned on, the customers were lined up and their identification checked. Those without identification or dressed in full drag were arrested. Women were required to wear three pieces of feminine clothing, and would be arrested if found not wearing them.
(This, of course, is just another example of the corrupt and unjust parts of the legal system that still exists within the US today.)
But the riots and protests continued into the night, being dispersed several hours later. But the protests continued the next day, with thousands of people gathering on Christopher Street in front of the inn and protests also continued the day after that.
A year later in 1970, the Christopher Street Liberation Day march was held to remember the events that had happened and people walked on Sixth Ave in New York City from Greenwich Village to Central Park. Other marches in different cities started to pop up as well. Although, this first march in 1970 wasn’t the celebratory parade that Pride currently is. One article about the evolution of pride by Yohana Desta states that:
Because of its celebratory nature, people often refer to the CSLD March as a parade, though it was always intended, and specifically called, a march. Its roots came from a somber place. Fred Sargeant, a man who attended the actual event, wrote a first-person account of the march for the Village Voice in 2010, writing that there were “no floats, no music, no boys in briefs.” Instead, they held signs and banners, and chanted “Say it clear, say it loud. Gay is good, gay is proud.”
There are two important figures from the history of the Stonewall Inn Riots that are often erased from the retelling. Sylvia Rivera and Marsha P. Johnson were both present at the riots (and some accounts name the then 17 year old Rivera as one of the first instigators to throw something at the police that day in June 1969). Both Sylvia and Marsha worked together to form the unfortunately short lived shelter called STAR House (STAR standing for Street Transvestite Action Revolutionaries).
Sylvia Rivera played a big part in the riots but also spent a lot of time advocating for those who continued to be marginalized when gay rights groups went more mainstream. David W Dunlap wrote about Sylvia after she died in February of 2002 and wrote about her struggle with the mainstream gay organizations:
Ms. Rivera often tangled with gay political leaders who favored a more conventional public front. When the Gay Activists Alliance eliminated transvestites from its civil rights agenda in the early 70’s, she turned on the group, Dudley Clendinen and Adam Nagourney wrote in ”Out for Good” (Simon & Schuster, 1999). ”Hell hath no fury like a drag queen scorned,” she warned. 
Sylvia worked tirelessly for people of color and low income queer and trans individuals and there is currently the Sylvia Rivera Law Project in New York City that does a lot of legal and educational work in memory of this wonderful woman. For more information on Sylvia, there is the ten posts for Sylvia Rivera’s Ten Year Memorial from several years ago and an article from the Village Voice calling Sylvia a woman for her time.
Marsha P Johnson spent time working with Sylvia on issues like the police raids, homelessness, and mentoring many youth. Some saw her as a mother figure and she often advocated for marginalized communities. Reina Gossett wrote an amazing tribute to Marsha on the Crunk Feminist Collective and quoted Marsha herself, saying:
In contrast to the equality movement assimilation strategies, Marsha P Johnson laid out a clear freedom dream during her interview “RAPPING WITH A STREET TRANSVESTITE REVOLUTIONARY” with Bob Kohler. She told Bob.
“STAR [Street Transvestite Action Revolutionaries] is a very revolutionary group. We believe in picking up the gun, starting a revolution if necessary. Our main goal is to see gay people liberated and free…We’d like to see our gay brothers and sisters out of jail and on the streets again. There are a lot of gay transvestites who have been in jail for no reason at all, and the reason why they don’t get out is they can’t get a lawyer or bail.” 
[Video of the documentary Pay It No Mind, where Marsha and others are interviewed specifically about the life and work of Marsha P Johnson.]
Unfortunately, Marsha died in 1992, shortly after Gay Pride. The police controversially rule her death a suicide but many have advocated that Marsha was the victim of a hate crime and that her death was in fact a murder. Her case was reopened in 2012 but I unfortunately haven’t found anything about what has happened with the case since then.