Late spring to early summer is the best time, when all plants are up and, for most tame pastures (with a lot of tame forages), the best time to clip is when grasses are in their 3rd to 4th leaf stage, and not quite pushing up a seed head. For fun and for a neat comparison, you can also clip later in the year when plants have headed out and started flowering, just to see what kind of differences you get.