We're providing a map (~2MB PDF) of Harvard Square, for your not-getting-lost pleasure. Here's how to find the church, which is where the ceremony and reception will be happening:
Once at Harvard Square, follow the signs to Church Street (also the 86 and 66 buses). The church will be immediately visible just across Church Street.
The adult fare is $2.00, and you can purchase CharlieTickets at any station kiosk. For more information on the subway, visit mbta.com.
As the First Parish Church website says, "Harvard Square is a maze of one way streets and is easiest to navigate with a map." But if you'd like to attempt it, here's how to get there:
From the Massachusetts Turnpike (I-90), take the Cambridge/Brighton exit 18. Stay right through the toll booth and follow the signs to Cambridge. (You'll be on River Street at this point.) Just after the bridge over the Charles, turn left onto Memorial Drive. Keeping the river on your left, follow it through two sets of lights, then turn right onto JFK Street at the third light. Turn left at the first light, onto Eliot Street. Go straight through the next light onto Bennett Street. Bennett dead-ends at University Road; turn left on University, and you will immediately see the University Place Garage on your left. (It's just about where Legal Sea Foods is marked on our map, with the legend "Le".) Parking is around $17 for the day, and the church is a few blocks away.
To walk to the church from the garage, follow University back the way you came, until it dead-ends at Mount Auburn. Turn right on Mount Auburn, then left on Brattle a block later. Brattle will snake around a little bit and take you directly to the church.
(Note: Google Maps, for some reason, thinks you can't travel on Memorial Drive from River to JFK. You can. Fear not.)
If you are coming from the Atlantic Ocean/Boston Harbor, travel upriver, and stop before the Larz Anderson Bridge.