Help me understand "picture dates"

I've been an RNAC for 9 years, with the majority of those years spent managing the Medicare/PPS caseload. Now, I am doing primarily the long-term, non-skilled residents. I've never gotten any "real" training/instruction on the CMI. I understand the reimbursement MDS items important for case mix. I'm in PA, so only the full assessments count for the CMI. I know our "picture dates" are in February, and I THINK August?? What assessments do these picture dates capture...the ones in the prior quarter? Sorry for sounding so uninformed, so please tell me all you know!