Now.

That wouldn't work well here in August but maybe it varies according to where you live.

If its a smaller sized plant you can dig it up and keep it in the shade a bit to adjust and prevent shock then replant but anytime I have tried moving a large more mature grass of any kind in summer it dies back. At first it looks OK but then it dries out on top, looks stressed then the leaves die back. It takes some time to recover the roots and you need to keep it watered until it settles in. Some grasses such as the Muhly types need a whole season to recover back to their former glory, I moved two good sized M. riverchonii in early spring that have not looked like much this season, lots of dead looking cut stems from last year and only a bit of new growth. Its best, not to mention easier, to move grasses in early spring before new growth begins after they are cut back low. I wouldn't attempt moving a blooming grass at all, especially in August.

I've observed that some types of grasses resent being moved more than others but large established ones seem to always resent it.