National Postcard Week is the first week in May.
"National Postcard Week began in 1984 with the idea that the International Federation of Postcard Dealers and different clubs would make a postcard to celebrate the postcard and promote their hobby.
To celebrate the week, postcard collectors either design and make their own postcards or purchase them to mail to friends and family. Always being sure the words National Postcard Week and the date are on the Postcard." [Source for more]
Additional Info links:
Too bad that great postcard magazine, POSTCARD COLLECTOR, went out of business. They kept you so well-informed on the postcard hobby (DELTIOLOGY), but luckily the Antique Trader has their postcard article archive online.
I really enjoyed the article on ads advertising postcards from publishers to retailers. Here are some pix from the article on ADVERTISING POSTCARDS, showing ads from a 1906 trade publication sent to encourage businesses to sell postcards.