I planted rye and clover about a month ago with an application of 10-20-20. What would be the ideal time to add nitrogen to give the rye a boost for a mid-December hunt?

You are probably going to broadcast granular Nitrogen Im guessing so give the N enough time for the rain and water to dissolve the Nitrogen and become available for the plant roots, since you are also planting a legume that will help the rye out b/c legume fix their own nitrogen. If you are incorporating the N into the soil you need to wait after the soil temperature is below 50 degrees, if the soil temp isnt below 50 the N will leach out of the soil right away.

KS,
I am broadcasting Urea. To be effective in December, do you recommend apply right away?