I am on "spring break", but there's been such a hullabaloo over what to wear to these visit weekends that I figured I'd come out of hiding to give my two cents.
You need two types of outfits: the business casual, and the casual. The exception is if someone states that the event is either formal or totally casual. Business casual means button-down shirt or sweater, belt, nice pants and non-sneaker shoes for men, and slacks or skirt, nice shirt or sweater, and dressier shoes for women. Everyone I've ever seen at a recruitment weekend was wearing some combination of these items. The casual is in case they have a party or a dinner with students, or if you decide to go out with some prospective cohort members.
The ideal situation is to bring mix-and-match items. One visit I went on had the meetings with professors and the student party on the same day, so I just wore my sweater all day and put on jeans in the evening. You pick up on these sorts of things after the fifth visit. To save you from some trouble, I've compiled a short list of the biggies.
Some specific advice:
1) BRING A BACKUP: I once went on a visit where the weather was so humid, my linen pants re-wrinkled and, since I was staying in a dorm room, I had no way to iron them. So I wore my backup skirt instead. I have also seen people spill wine on their pants at the opening reception and then have to wear the winey pants the next day to the general meeting. You don't have to be that person.
2) LAYER: Schools in cold climates tend to be uncomfortably warm inside. You wouldn't think it, but schools in warm climates are also freakishly warm inside, because everyone there still thinks it is freezing. Bring a cardigan, sport coat, pashmina, whatever you've got so you aren't shivering through the meet and greet, and for goodness sake don't wear a huge sweater with nothing underneath.
3) BRING SENSIBLE SHOES: Planned or unplanned, there is always a campus tour, so this isn't the weekend to break out the new, unworn shoes or the stilettos. Also, whenever things run late you'll be expected to book it to the next event. Nursing a blister is not an option. I once ran full-speed through a building to arrive on-time to a meeting with a Big Wig, and I was never so appreciative of ballet flats.
4) DON'T GUESS: If you're still unconvinced, don't be afraid to ask the graduate coordinator. They have answered far stranger questions than "What is the dress code?".