[Roanon]

Originally posted by Slick:
Combat can occur any day that there is movement.

More precise, AFTER any movement portion of a day. So if one fleet moves to intercept and enter another fleet's sector, combat will not occur if that fleet moves away during the same day.
There are some very odd effects of this mechanism, for instance if one fleet A wants to intercept another, equally fast, fleet B which is trying to move past the point where A is starting, A will never capture B if the number of intervening sectors is even. Experienced players counter-trick this by moving a single sector sideways before issuing the attack order, or by including a slower ship in the intercepting fleet so movement will not take place on the same days for both fleets.