Googled: "decomposition," "stages of decomposition," "decomposition mass graves"
Setting: Post-apocalyptic U.S.
Scenario: A massive disaster kills millions of people in a single day. I assume they'd have to be buried in mass graves, for the sake of sanitation. The deaths (which range in terms of goriness from "looks like he/she is sleeping" to "entrails scattered across several city blocks") happened in November; it's now a rainier-than-usual March.
My morbid question is this: How decomposed are the bodies now? All I've managed to find out is "it varies according to environmental conditions, embalming, etc."