11:59 PM on the 26th is the answer to your question. However...

While the hold time is always 11:59 PM, it varies by day depending on a lot of things. The fare type, how far you are from departure date, where you were when booking this fare. etc. If the fare is a discounted 21 day advance purchase fare and today is 21 days out, then the hold would expire at 11:59 PM tonight for fares booked and held today. If however, it is a discounted 21 day advance purchase fare and today is 22 or more days away from your departure date, then fares booked and held today would be held until 11:59 PM tomorrow night. Full fares, and oneworld RTW/CirPac fares can be held much longer. Bookings for award tickets can be held 5 days I believe. I believe the time and day you book the ticket, and the time it is held until is relative to CST, but someone will correct me if I am wrong. Hopefully the information above was of some help.