I think you must have the date wrong. If they plan to arrive on the date of the closest approach of Mars, they will have taken a longer time to get here than is sensible. What you need to to do is find the speed of their craft, and then arrange for them to cross the midpoint of their journey on that date, and then work out the time of the journey and where the Earth would be ... Oh bother, I think this involves calculus and no-one will tell me how to do it. Some sort of iterative process best done by computer or people with too much time on their hands. But they won't be here on the 27th!