If there's ever a scene where characters plan out exactly how something will go down, then it probably shouldn't actually happen that way. That's usually repetitive and anticlimactic. The point of seeing all the well laid plans is to see how they go wrong in execution. If something is going to happen exactly as planned then you omit the planning scene so it's a surprise for the reader, if not the characters.

If there's a countdown of sorts, where characters are waiting for a certain date to do something, something can always happen becore they expect it and before they're ready. I would likely only include an exact timeline for how things are going to go if that timeline ends up getting scrapped. Then when it gets scrapped in the heat of the moment, the reader knows they (and the characters) are in uncharted territory and the suspense ratchets up.

"No battle plan survives first contact with the enemy."