The web site is pretty striking - sort of takes your breath away at first glance. Down near the bottom are links to Las Vegas Conventions, Trade Shows and Special Events Calendar by month including attendance levels and location. When I was a tourist I used to use this info to find slow dates to come visit Vegas - cheaper rates

Good Luck and happy hunting

Here's the official convention calendar. While it doesn't show recurring conventions, I think based on the size of the convention might be a good indicator whether it is annual or not. Hope this helps!

Here's the one I use...

...similar to the other recommends and, obviously, the same info.  I just like the format a little better.