You'd want to leave warm-season bunchgrasses but try to keep down cool-season turfgrasses. The usual method is burning or mowing in late fall or mid-spring. Don't burn or mow in summer, as that's when prairie grasses and forbs do their growing.

Without grass, its not a meadow? That recommendation doesn't make sense to me.

You definitely wouldn't want to spray right now, since you'd kill all of the emerging flowers. Perhaps the recommendation you saw was to eliminate non-native, cool season grasses, which can be a pain in a meadow, since the grow earlier than most wildflowers and compete with flowers for space and resources like water and nutrients. Native, warm season grasses are usually considered a desirable part of a meadow. Regardless which one you have, now is not the time of the year to try to remove grasses.