...de la News Journal:
Uncle Sam Day was adopted by Congress in 1989 and is recognized on Sept. 13. Two theories surround the origin of this holiday. According to Matthew's site.
The first points to a man names Sam Wilson, who was refered to as "Uncle Sam" duing the War of 1812 after he brought soldiers barrels of meat. Congress issued a resolution recognizing and established a monument in his hometown of Troy, N.Y.
The second theory says that in Irish immigrant native language of Gaelic, the United States of America is translated as "Stait Aontaithe Mheiricea" or "SAM".