Q: I have my big _____ Exam coming up and I can take it on March 12, March 21 or on April 21. Which day should I take it on so I can pass?
A: The ONLY thing that can help you pass your exam is study and preparation.
Do everything you can to prepare - that's it - the day doesn't matter.
If you have your heart set on picking a day that feels lucky to you, to help you feel more confident, and help bring out all that study and preparation that you are going to do (you ARE going to study hard, aren't you?) - there are a couple of things that can help...but not really tarot.
If you feel drawn to numerology, you could look at it that way. Both March dates are "3" days which tend to be lucky, especially if they are related to something child-like, creative, playful or the arts.
The April date is a "4" day, almost the exact opposite quality from a "3" day in that it is very work and stability oriented, has to do with memory more than creativity, is very solid and reliable.
Numerology isn't my strongest suite...so I suggest researching it on your own to get a feel for which day you might choose.
Another way to choose is consult a chinese astrology almanac, and see which day has the most of your lucky element. You can find out what your lucky element is on websites like http://www.chinesefortunecalendar.com
No matter which day you pick...the future, and your test results, are up to you. STUDY STUDY STUDY!!!