The best way to get around is to get a rental car, but if you prefer not to, here are some alternatives. (You would still need to get a taxi from the train station to your hotel and arrange your own transportation for the wedding and reception.)

Trains

Trains to this area leave from Grand Central Station on Metro North Railroad.  The Harlem line (to Southeast) has stops in Mount Kisco, Hawthorne and Katonah.  The Katonah Station is walking distance to the church, but you would need a taxi to get to the hotels from the other stations. 

For current trains see:

http://as0.mta.info/mnr/schedules/sched_form.cfm

For pdf schedules (not yet current,click on Harlem line)

http://mta.info/mnr/html/planning/schedules/schedules.htm

From JFK: 
Take the airtrain to Howard Beach Station ($7).  Take the A line subway

($2) to Manhattan and get off at Fulton Street/Broadway-Nassau Street stop.   Transfer to the 4,5, or 6 Uptown and get off at Grand Central Station.

Or, for $15 you can take a bus directly to Grand Central Station - more expensive, but a lot easier, especially if you are unfamiliar with the subway system. 

From Islip:  Take the Long Island Railroad (LIRR) to Penn Station.  From here you can take the A,C, or E subway Uptown to 42nd Street, and transfer to the shuttle (S line) to Grand Central Station.

Metro North information: http://mta.info/mnr/index.html

Subway map: http://www.mta.info/nyct/maps/submap.htm

Buses, Shuttles, and more:

From JFK: http://www.ny.com/transportation/airports/JFK.html

From LGA: http://www.ny.com/transportation/airports/LGA.html

(See the Westchester sections at these websites)

Google Local directions:

From Holiday Inn - Church - Reception cite - back to Holiday Inn: CLICK HERE

From Comfort Inn - Church - Reception cite - back to Comfort Inn: CLICK HERE

           If you are using LINUX, download the pdfs here:  holiday_inn_saturday.pdf

                                                                                                comfort_inn_saturday.pdf

Interactive Map of important locations: CLICK HERE

Note: When returning to the Holiday Inn at hwy 133 and seven bridges road there is a dangerous intersection. Going toward the Holiday inn (to the east on the map). There is a downhill and sharp curve right before the stop light at this intersection. The distance from hwy 100 to seven bridges road on hwy 133 is ~2.3 miles.