Everything is a guess if you want to take it to the extremes but estimating based on experience and historical patterns is far more justifiable.     It should be observable that gains are far more likely to occur after halvening then before. 

If you consider the actual change itself, a constriction of supply though its increasingly slight now vs the first early halvening.   The changes are after the event, each new block gives a lower block reward so only after a year of that change can we say it adds up to something bigger then a tiny change in BTC expected circulation.

Simple advice applicable to almost any market based price is buy the rumor sell the news.   If you simple transferred that advice here you would sell the halvening.   People who've been around will tell you not to now because most gains will accumulate after in that change