Letter-Writing Day (Fumi no Hi) is a tradition originally founded by Japan Post back in 1979 to promote the fun of exchanging letters. It is celebrated on the 23rd day of every month and most of all on July 23rd.
"Fumi" derives from the words for 2 (fu) and 3 (mi), while "Fumizuki" is an old name for July in Japanese. These two Kanji also mean "letter" and "month" — so it is literally THE perfect day to write a letter!
Japan Post sells this stamp series every year. The stamps of the year 2014 were issued to commemorate the letter writing day on July 23rd. The designs of stamps are floral patterns which were suited for all seasons.