Per Wikipedia, "In the United States, it is celebrated in October to coincide with National Coming Out Day on October 11. In the United Kingdom, it is observed during February, to coincide with a major celebration of the 2003 abolition of Section 28."
In the UK, you can learn more at http://lgbthistorymonth.org.uk/.
Some of my favorite picture book titles are And Tango Makes Three, Heather Has Two Mommies, My Princess Boy, King and King, 10,000 Dresses, I am Jazz, The Boy Who Cried Fabulous, and Uncle Bobby's Wedding.
Lee Wind also has some great reading lists for all ages at his blog, I'm Here, I'm Queer, What the Hell Do I Read?