depends... who's track day are you doing? each track owner / day organizer has their own set rules reguarding gear. For example, Tony's Track Days doesn't require leathers... here's their guidelines (minus the rules about helmets)
GLOVES - Full leather gloves that are in excellent condition and leave NO EXPOSED SKIN on the hands or wrist.
BOOTS - Over the ankle boots, preferably motorcycle specific. HIGH TOP SNEAKERS are NOT ALLOWED, and we discourage the use of "work boots".
LEATHERS - Full leathers are not mandatory, but are highly encouraged. If you don't own leathers, there is one set available for rental. It will fit a person 5'6" to 5'8" with a small build (up to 145lbs or so). Fee is $50 for the day with a $100 damage deposit. Email me PRIOR to the date to make arrangements - tony@tonystrackdays.com
Kevlar type suits or jackets are also acceptable.
Mesh type jackets are not allowed unless they are reinforced in all impact areas and have a back protector. I would suggest emailing me with a photo or link to your particular jacket if you are in doubt.
NOTE: If you don't have leathers or some other kind of full riding gear, you can ride in jeans and a leather jacket. If you do this, please "double up" on the jeans or have some other form of hip/butt protection underneath. You must also have some sort of knee protection. "Rollerblading" type pads or even knee pads from a "Home Depot" type store will work.
The track day alot of us are doing down at VIR, however, has much more stringent requirments.