By definition, it is either true now or it is false now that the slide will be finished tomorrow.
The fact that something is true doesn't imply that we know it is true.
A fact is a state of affairs we know to be the case. It is certainly not the case that anyone would know for a fact that some slide will be finished tomorrow. We don't even know that there will be a tomorrow to begin with.
We don't know the future, as far as I am aware, and so we don't know future facts, and we don't even know that there will be a future to begin with.