January 21-26.
Unfortunately, unlike its European counterpart Musikmesse, NAMM isn't open to the public. You have to be a member of NAMM (mostly music retailers and vendors), an exhibitor, performer, press or an invited guest of one of the above.
Despite it being a closed affair, you do see lots of gatecrashers there, especially on Saturday. Some are groupies on the hunt for rock stars to harass, some are wannabe rockstars whose parents have connections. Every exhibitor has a (limited) number of badges to give out to their favorite customers and supporters. Contact a vendor that you've had a long relationship with that's also
exhibiting at the show and ask. This year at least one software developer (no, it wasn't Cakewalk) sent out emails offering NAMM badges to their customers. Musical educators and students can get passes, too.
There is also an admission fee for non-NAMM members. In the past it was $25, but nowadays it's between $50 and $100 depending on the type of badge.