This is from The Library of Congress:
Lesbian, Gay, Bisexual, Transgender and Queer (LGBTQ) Pride Month is currently celebrated each year in the month of June to honor the 1969 Stonewall riots in Manhattan. The Stonewall riots were a tipping point for the Gay Liberation Movement in the United States. In the United States the last Sunday in June was initially celebrated as “Gay Pride Day,” but the actual day was flexible. In major cities across the nation the “day” soon grew to encompass a month-long series of events. Today, celebrations include pride parades, picnics, parties, workshops, symposia and concerts, and LGBTQ Pride Month events attract millions of participants around the world. Memorials are held during this month for those members of the community who have been lost to hate crimes or HIV/AIDS. The purpose of the commemorative month is to recognize the impact that lesbian, gay, bisexual and transgender individuals have had on history locally, nationally, and internationally.
In 1994, a coalition of education-based organizations in the United States designated October as LGBT History Month. In 1995, a resolution passed by the General Assembly of the National Education Association included LGBT History Month within a list of commemorative months.
You might be interested in The Best Ways To Support LGBTQ Students.