Timing depends on what you're wanting to plant. Seeds have ideal germination temperatures. I use a compost thermometer to check my soil temp.
In MS, you should be able to sow several types of plants right now and be just fine. It'll be too late for plants that need to reach a certain stage of...
To keep your soil food web active, you could plant out something like Crimson Clover. Then turn it into the soil after it matures for a nitrogen rich soil come spring. Or, plant out something to eat like greens and turnips.