If you are using butter instead of pan drippings you can definitely make it ahead of time.  It will keep in the refrigerator for 2 -4 days.  It only takes about 15 minutes to make so if you are using pan drippings it is easy to make as the turkey or roast is sitting for a few minutes before carving.  You can also freeze the easy gravy for up to 3 months in an airtight container. Thaw in the refrigerator for a day before use.