Online reservation systems for Ryanair will be unavailable from 5 pm on Wednesday until 5 am the next day for an "essential" upgrade, the airline said.
So far, there is little detail as to what the 12-hour upgrade will involve, but in May the airline announced it was moving almost all its infrastructure to AWS from Microsoft.
As services on www.ryanair.com and its app will be down, the company is asking for customers travelling on Wednesday and Thursday to ensure they check in online and print out boarding passes by Tuesday 6 November.
However, the airline usually charges 55 per-person to check-in at the airport and 20 to reissue a boarding pass. This will still be in force and passengers travelling between the 12-hour window that don't book online by Tuesday will be subject to these charges.
Ryanair's Robin Kiely said: "As part of our continued digital development, a web closure and system upgrade will take place for 12 hours, from 17.00 on Wednesday 7 November, until 5.00 on Thursday 8 November, during which time our online check-in service will not be available.
"We sincerely apologise for any inconvenience caused by this upgrade, which will further improve Ryanair.com, Europe's largest travel website."
The airline normally takes some 15,000 bookings an hour and it hasn't given much time for the 350,000 passengers, who are already set to fly on Wednesday and Thursday to check-in online and print off boarding passes.
Nick Ford, chief evangelist at Mendix, said that although upgrades were essential, they were by no means easily managed.
"The interest in Ryanair shutting down its website and mobile app for 12 hours goes to show how business-critical applications really are," he said. "The cost of 12 hours 'downtime' quickly stacks-up, from lost online bookings to needing more on-the-ground check-in staff.
"While it's easy to say businesses need to swiftly roll-out, migrate and update apps, it's quite another thing to deliver. Migrations and upgrades require organisations to build a system that remains flexible to adapt to changes."
