Whatever date your online account says you're eligible to upgrade on is correct, whether it's the actual expiration of your contract, or your 20 month "early upgrade" period.
If buying directly from Verizon doesn't matter much to you, Wirefly has pre-orders available for $179.99 with contract renewal. Your early upgrade will work over there just fine as soon as you become eligible. I almost pulled the trigger, but in the end, I just went with a tried-and-true Verizon pre-order. Not that I don't trust Wirefly, but, as pointed out to me by another member, any device issues and whatnot will be handled through them and not Big Red. I'd rather just keep everything in house, even if it costs me an extra $20 ($70 until the rebate comes through).