Summer training, which generally includes a week-long regional induction, five-week intensive training institute, and a week-long regional orientation, is mandatory, no way around this one. As a general rule of thumb, once corps members begin induction, they can expect to have weekday commitments from that day forward until the beginning of the school year.
If you have a conflict that is earlier in the summer, you may want to preference regions with later training dates. View the 2017 summer training schedule to get an approximate idea of dates. The 2018 summer training schedule will be available in early spring of 2018.
If you know now that your conflict would require you to miss more than one day of each region's training schedule, then you will not be able to join the 2018 corps. Please consider applying for the 2019 corps instead.