There are X fares (fare basis XHQJ95M3 main cabin, XHPJ95B3 basic economy) available for purchase for MSP-BCN flights on May 27th. However, these have a 90 day advance purchase requirement (see fare rule summary below). Today, Feb 26th, is exactly 90 days out from May 27th. So there's not much question fares will go up somewhat if you don't purchase by today as that X fare will no longer be available due to not meeting the advance purchase requirement. From fare filings on Expert Flyer, the next fare up from this X fare is a T fare with a 50 day advance purchase requirement. It's only $50 more, so it's a fairly modest fare bump. There's the possibility DL will load new, lower fares at some point after this weekend. But that would be your only hope of getting lower fares than what is available today at this point. If you wait until tomorrow to purchase, you could still potentially book an X fare for May 28th departures (since it would still be 90 days out), but not May 27th.