People eat tangyuan for lots of reasons: tables for family reunion dinners are round; the moon is round; and, as usual, the easiest explanation is just, "it's good fortune." Tangyuan is a southern thing though. In north China, they eat yuan xiao (元宵), a bigger, harder dessert rice ball. Anyway, tangyuan are really good -- grab a bowl at your local Chinese restaurant or buy a bag at the supermarket and boil them yourself.
Shanghai has two major Lantern Festival events this year. The bigger one at the Expo Park runs until March 13, with gigantic lanterns that might break records. You can buy tickets on Gewara from 110rmb for tonight.
For more old-school vibes, Yu Garden's Lantern Festival runs for three days until February 25, with a huge Monkey King lantern installation and smaller ones hung throughout the Old City's upper floors.
(Obviously no fireworks at either location this year.)
Even though it's raining today, expect big crowds, extravagant decorations, and the final wave of the holiday spirit. Around 7-8pm should be a good time to show up. Even if you don't, take a break from that WeChat moon to look up at the real moon. Happy Yuan Xiao Jie.