I think the impact on the bitcoin price is not easily predictable.
At least in theory, a known event should not have an abrupt impact on the bitcoin price at that time, because all parties (bitcoin market participants and miners) reflect that information in advance when weighing up the economics of their current decisions. There may be an action required at that point (such as miners turning off equipment), but, again in theory, the potential for all those actions and their consequences should be built into the current price in a fully informed market. In fact the entire future schedule of reward reductions should, in principle, already be built into the market's valuation of bitcoin in this way.
That means that any resulting movements that do occur at such times are not directly from the event itself, but from endogenous market reactions as talk of the event becomes elevated and market participants try to collectively trade off the sentiments of all other players and less informed participants. This endogenous action may (or may not) trigger some increased volatility around the event, but, again in theory if the market were efficient (a big "if") at pricing in the events, it would not be possible to profitably predict the price reaction one way or the other.
Having said all that, the bitcoin market is very immature, so in practice it may be possible to speculate on whether the market is actually pricing in the event efficiently or not.