During the halving, the block rewards (number of bitcoins) produced is cut in half. Meaning that the inflation rate of new coins is cut in half at every halving. The current one dropping from 12.5 to 6.25 bitcoin created every block. That is significant in that miners are considered the largest sellers, and the halving event likely means they are going to have less supply to sell.

The market has gone up both times during the last halving, and it appears to be doing the same again.