Times article

QUOTE
Ryanair is shutting down its website for three days in February which, it is estimated, could cost the airline £20 million.
The no-frills carrier is shutting down for 74 hours from 22:00 on February 22 to 23:59 on February 25 while it changes to a new reservations system.
While flights will continue as normal over the three days, no bookings will be taken via the website or over the phone.
...
The shutdown also means that customers with Ryanair bookings will be unable to make changes to their flight booking, including date changes and passenger name changes over the 74-hour period.

If you use Ryanair, be aware of this downtime and plan accordingly.