It’s quite common for older game conventions to use a roman numeral in the name of a convention, noting how many times that particular convention has run since its inception. It appears that the majority of newer cons instead use the current year. PolyCon XXX versus PaizoCon 2012 for example.
Here’s a quick reminder on how roman numerals work:
I = 1
V = 5
X = 10
D = 500
S=1/2 (although I've yet to see a Con use this)
There is no concept of decimals in Roman numerals so 1.5 is impossible, but 1½ is not.
When using these to create a number it is generally accepted that you can't repeat the same roman numeral more than three times (in a row) and that you can't subtract from a number more than two places larger (i.e. I only subtracts from V and X, not L-M). So 49 is XLIX not IL.