Bitcoin halving is a part of the bitcoin protocol that occurs every about 2-3 years ( it's set to be triggered automaticly by a specific amount of blocks created by miners(people running the network) hence the variation in time occurence when it happens the reward the miners receive for creating a block is halved in order to maintain the deflation model of bitcoins ecosystem causing mining operation costs of creating bitcoin to go up which is supposed to also push the market value up ( nobody unless forced by negative circumstances e.g. financial need will want to sell at a price lower than production cost)
Also means many outdated minning farms either will go bankrupt or need to invest in upgrading their rigs which often creates sell pressure from the miners which could explain the initial dips in price which happened just around previous halving events before the price went up