Ah, the Fourth of July--America's favorite holiday. Almost everyone has rustic memories of fireworks, picnics, and typical holiday hanky-panky. You may be far from the scene of these memories, stuck with an urban Fourth, but you can still have some fun. If holiday celebrations are your thing, then Beantown has got a doozy for you. On the evening of the great day, there's a traditional celebration at the Esplanade, downtown on the river basin. Two years ago 400,000 people jammed into the park to frantically celebrate the Bicentennial; while there probably won't be quite that many people around this time, if you're going to go, get there early. Arthur Fiedler, the cute, crusty conductor of the Boston Pops Orchestra, those refugees from classical music, will be on hand with his boys for the umpteenth year in a row to make merry music. The selections will feature all the patriotic standards. Bring plenty of wine or what have you, sit back and enjoy the show. The Fourth, after all, only comes once a year.