It works like this:
Priority goes to the first day the waiver would be completed.
If you have multiple waivers, then it works according to what you set.
In your case, it will take the 25th first. If you get it, your priority goes back to last in your league. If you really want the one on the 26th, you need to cancel the one for the 25th. This makes sense, because someone could put in a claim on the last day. The system can't look at the waiver for the 26th and know for sure if you have gotten it or not.