every birthday should be important, because your birthday is the celebration of your life. And for your friends and family, it's the day they all tell you how much they love you being alive and well if they don't do that everyday all ready.
but it's 21st birthday that's the real shit, just avoid the 21 shots tradition. as fun as that sounds it should normally lead to certain death via alcohol poisoning (21 shots of 40% alcohol, seriously, think that one through, can one normally handle 10? imagine 21, it'll replace a quarter of your bloodstream). but take your time enjoy at most 8 or 9 shots with your chums, 7 if i did my guesstimation math wrong. it's all based on how big you are. the bigger you are, the more you can handle at a linear scale (that's important, linear scale, not exponential scale)