Although any March 14th is Pi Day, here is a partial list of some of the "most" Pi Days in Base 10 in order of precedence, in my opinion:
Base 10 M/DD/YY, M/DD/YYY, or M/DD/YYYY format:
- March 14th, 1592 AD (or BC) - known as Ultimate Pi Day: the largest correspondence between calendar dates and significant digits of pi since the introduction of the Julian calendar (according to Wikipedia).
- March 14th, 159 AD (or BC) - debatable: 3/14/0159 is less Pi because of the 0
- March 14th, 15 AD (or BC) and 16 may also be acceptable if you round up
- March 14th, 1 AD (or BC)
- March 14th on any year ending in 15 (or 16 if you round up), e.g. 3/14/15 (or 3/14/16)