According to www.holidayinsights.com:
"National Pig Day recognizes and gives thanks to domesticated pigs. For some unknown reason, big, pot-bellied pigs seen in zoos, are often the symbol of the day. Pigs are clever and intelligent animals. But, most people are unaware of this high level of intelligence. They can be taught to do tricks. Some people even keep them as pets. Today is a day to give pigs the respect that they deserve."
"Ellen Stanley, a Texas art teacher created National Pig Day in 1972. Her intent was to to recognize and be thankful for pigs as intelligent domestic animals.
There is no evidence to suggest that this is truly a "National" day, which requires an act of congress."