A general term for a big ol' Southern gathering. May involve both family and friends. Always involves copious amounts of food (often potluck style), and hopefully some form of alcohol, be it PBR or moonshine.
A "holy hootinany" is slightly different. Similar to a regular hootinany, this is a social event often sponsored by your local Southern Baptist congregation. The main difference: replace PBR with preaching.
"Yeah, all our kin are gettin' together for a little covered-dish hootinany this Saturday. Cousin Fayleene's bringing that god-awful casserole."
"I got saved last Sunday afternoon, at that church barbeque. That Bible-thumping preacher scared the hell right out of me. Then they fed me till I about half died. That was a holy hootinany for the books."