This is just an observation but the Sept contract @ the cash close was neck and neck in terms of price and this is only the middle of July, so I checked the Dec's and same thing only a point or so difference. Isn't that odd? I always remember a large prem to the ES...

The spread tends to drop, relatively, as interest rates fall and hence carry costs fall, and I've seen some futures trade at a discount at times.

I've noticed the premium being narrower in bear markets and wider in bull markets as well. Sometimes, but rarely, at a discount and only in fast moving markets and back to a premium when the market closes. As Jim said the premium reduces in a linear fashion by the fraction of days left to expiration.