From its mysterious beginnings 20 some years ago, the SICPP music festival has been delighting, confusing, and enrapturing audiences in Boston. The Summer Institute for Contemporary Performance Practice (or “Sick Puppy,” as it’s known to regulars) started as an intensive performance seminar on the piano music of the 20th century. Over time, Sick Puppy expanded to invite musicians of all instruments, and started a composition fellowship and a workshop for electronic music. In its current form, Sick Puppy is a 9-day celebration of the very best of 20th- and 21st- century classical music. Beginning on Sunday, June 17th, faculty and guest artists perform free, nightly concerts in New England Conservatory’s Jordan Hall.