If you import a schedule you can choose the Opening Day. If you don't have a schedule to import, just export the one you're using right now, and then import it again.
Thanks, Spartacus, for the suggestion. That makes a lot of sense.
I don't have a schedule to import, so I tried the exporting option. But when I export the schedule, it exports as a .psc file. What software do I need to open that, edit it, and then send it back to Puresim? I got stuck on that step.