During the 20th century, ST. PATRICK'S DAY ditched its Catholic essence and gradually became a secular celebration of Irishness centered around green dye, beer, plastic shamrocks, and gazing wistfully at bagpipers. Atlanta's biggest St. P.'s throwdown is a four-day party at Underground Atlanta Fri.-Mon., March 14-17. Highlights include the city's 127th annual St. Patrick's Day parade Sat., MARCH 15, and the fact that bars at Underground can pour until 4 a.m. The lowlight will almost certainly be your headache on March 18. Free. Parade: noon. Peachtree Street and Ralph McGill Boulevard. Festival: Times vary. Underground Atlanta, 50 Upper Alabama St. 404-523-2311. www.underground-atlanta.com.