You have to be careful using Wikipedia as a source, but in this case it has the best explanation. It's a detailed one that I will try to summarize for you, Alexandria.
In the Southern Hemisphere, winter falls in July, so countries such as Australia, New Zealand and South Africa have Christmas events at that time in order to have a winter feel to them. Yes, they also celebrate Christmas there in December, despite it being summer.
For us in the Northern Hemisphere, stores and nightclubs sometimes throw "Christmas in July" sales and promotions in a way to bring the atmosphere of Christmas but with warmer temperatures.
But its origins go back much further.
In the English translation of an 1892 opera called "Werther," a group of children rehearses a Christmas song in July — to which a character responds, "When you sing Christmas in July, you rush the season."
The earliest known occasion that the phrase was used literally was in July 1933 at a girls' summer camp in North Carolina. They celebrated with a Christmas tree, gifts and a visit by Santa Claus.
Two years later, it got some national press when the National Recreation Association's journal "Recreation" described what a Christmas in July was like at a girls' camp. In 1940, a Hollywood movie called "Christmas in July" was released. In the story, a man (Dick Powell) is fooled into believing he has won $25,000 in an advertising slogan contest. He buys presents for family, friends and neighbors, and proposes marriage to his girlfriend (Ellen Drew).
U.S. Army, Navy and Postal Service officials, in conjunction with American advertising and greeting card industries, threw a "Christmas in July" luncheon in New York in 1944 to promote an Early Christmas Mailing Campaign for service men and women overseas during World War II. It was repeated in 1945.
I couldn't find anything about Christmas in June, but in the 1950s, Yellowstone National Park hosted a Christmas in August celebration to accommodate an annual performance of Handel's Messiah.
To read more about it and links to more information, click here.