If you really look, you can find a "holiday" for every day of the year. Obviously, the majority are not major holidays or are celebrated by everyone. Here are some I found for August that I thought were interesting.
National Ice Cream Sandwich Day
Book Lovers Day -- encourages you to find a shady place to relax with a good book.
National Aviation Day -- in honor of Orville Wright's birthday. The day honors the accomplishments of the Wright Brothers. It's sometimes called Wright Brothers' Day.
Be an Angel Day -- encourages people to do an act of kindness and to help others. Jayne Howard Feldman, the creator of this day, says she was inspired by angels.
Frankenstein Day -- is in honor of author Mary Wollenstone Shelley who was born on August 30, 1797. She wrote Frankenstein in 1818.