Chaim, Field Days are chok vlo yaavor always on the last full weekend in June, azoy shtait in ARRL rules. (Probably because they are the longest days of the year in terms of daylight hours.) The rules are that from 2 pm EDT Friday till 2 pm Sat they set up the stations. From 2 pm Sat until 2 pm Sun they operate. I usually go Motzaei Shabbos and/or Sunday morning until the end. This is the same everywhere in the country and in the world (except for time zone differences).
I used a mag mount for a while, but haven’t been on a repeater for a number of years now. At Field Day I usually operate CW (morse code), but I think there is some activity on 2 meters/70 cm at my club. But I am not sure if it is official Field Day contesting that counts towards the score. I have been to a number of clubs in different locations, and they will probably be glad to let you operate for a while, even as a guest. You may want to give them membership dues when you are there, if you plan on being active in that club and using their repeaters regularly.