LIA comment to JGMF "he informally played an acoustic with Rosalie Sorrels & David Bromberg as she sang 'If I Could Be The Rain' somewhere in the fields of Woodstock; thought to be on August 15"
Other reports have Garcia playing at the medical tent for the folks that took the brown acid, and with Marty Balin at the Merry Pranksters campfire, seems likely he was playing informally plenty at Woodstock.