If the fashion police were a real thing, John and I would both have been in jail a long time ago. Cocktail attire is fine. No tuxes or tails or ball gowns required, but if that's your thing, have at it. We suggest that you bring a jacket or sweater for the ceremony, though, because the Catholic Church frowns on underdressed people. (They don't want you to suffer needlessly in the aggressively air-conditioned building, right?)
Please see this link for the full answer per the US Conference of Catholic Bishops: http://www.usccb.org/prayer-and-worship/the-mass/order-of-mass/liturgy-of-the-eucharist/guidelines-for-the-reception-of-communion.cfm The short answer is that Catholics who are not in a state of grave sin and who have fasted for one hour prior to communion are eligible to receive Communion, while non-Catholics are not eligible (though we know you're wonderful people!), due to theological differences in the way that Catholics and non-Catholic Christians understand the Eucharist. All are encouraged to participate fully in the rest of our celebration, and we warmly welcome all of your prayers and support! Thank you for your understanding.
There are several lots dedicated to Basilica churchgoers, the most convenient of which is on Pitt Street between Duke St and Wolfe St (308 S Pitt St for GPS navigators). We recommend moving your car after the ceremony, as the church celebrates a regular mass at 5:00 that evening. Guests are not permitted to use the gated parking lot attached to the Old Dominion Boat Club, but there is a public parking lot right next door to that lot, and three other public lots within a block or two.
"What? It's good." [Goes back to swatting soap bubbles with tennis racket.]