It depends on ground temperature and dampness. The usual guidance in the UK is April/May, but I'm not sure how things are where you are. My rule of thumb is that if I wouldn't want to lay my bare belly on it, it's too cold/damp for my guys!
The area also needs to be secure and the run should be fully enclosed and predator-proofed, just in case. You also need to be sure there's no non-guinea pig animal urine on the grass as that can make them poorly, so public areas aren't recommended.