The Graceful Envelope Contest was created in 1995 to promote the art of calligraphy and the role of letters in binding people together. Calligraphers and artists from around the world are invited to participate in the 20th annual contest, conducted by the Washington DC Calligraphers Guild and the National Association of Letter Carriers. It's open to everybody, also children. This year's theme is The Superlative Letter "S", and the postmark deadline is March 24.
You can read here the contest rules.
Even if you decide not to participate, you can enjoy the past year's works. They are amazing!