According to
this practice matches start at 6pm. Practice fields are in the pits in St. Louis, so it's a safe bet they'll be open an hour or two before that to allow for inspections, load in, etc. For some reason 5pm sticks in my mind. I'm sure it'll be posted on the linked site sometime.
In previous years they've specified that only 5 people are allowed in from each team initially to do load in to keep crowds down and allow all teams to move their stuff without a lot of people in the way. I've been inside for the inspectors meeting the past two years and don't know what the latest rules are. Hopefully someone else can comment. Either way it may be a good idea to consolidate your stuff in a way that it can be transported / carried / rolled by 5 people or less making only one trip, even if that's not a rule anymore.