Since it's a known future event, the halving should not affect the price.
Bitcoin value is mainly psychological. I expect many people will start panic buying if they see the price start to rise near the halving which I expect it will. This can have a knock on effect too.
Mining activity, though, sees a big cut in revenue, and some miners are going to drop out. Miners presumably have run spreadsheets and know what gear goes to negative ROI after the halving, and will presumably have planned things so it more than paid for itself well before the cutoff date.
How do you know? Maybe due to the lower supply of bitcoins the price may rise?
We know that the block reward will decrease from 25 to 12.5 BTC per block, and it's reasonable to assume that rational miners not only also know this, but have factored it into their ROI calculations.
We know what the new supply of BTC will be each block - it'll be 12.5 after the block reward halves. What we don't know, but can attempt to extrapolate from current data, is non-new supply - long-term holders (for example) converting BTC to fiat, the vendor of those exciting new ACME widgets (which are only sold for BTC) needing to convert BTC to fiat to pay their suppliers, short-term speculators needing to close long positions, etc. We also don't know - but can attempt to extrapolate, guess, whatever - what demand will be like (people buying ACME widgets will create demand, etc).
tl;dr - it's all down to supply and demand. Do your best to calculate what will happen to supply after the halving, and to demand after the halving, and you have as good an answer as anybody else! Anything else is just noise, people's wild-assed guesses, predictions based on something that happened once before, etc.