Bitcoin blocks have a difficulty that adjusts every 2016 blocks. The halving is scheduled to take place once block 840,000 is mined. The difficulty is adjusted to keep the average time between blocks approximately 10 minutes.

The market will resolve 2016 blocks after the difficulty adjustment takes place; i.e. 

block 842,016. The market will resolve early once it is is mined.

If the diffculty of block 842,016 is less than the difficulty of block 840,000 this market will resolve to YES. If it is the same or higher than the market will resolve to NO. 