Budget airline Ryanair is closing its website for bookings until late on Monday to upgrade the system.
Ryanair says the work is for an essential system upgrade
Customers will be unable to book flights online or on the telephone, and its web check-in service will also be unavailable from 2200 GMT on Friday.
The system will return at 2300 GMT on Monday, 25 February, when the airline will be promoting some cheap deals.
The shutdown will allow it to comply with an order by UK market regulators to make prices on its website clearer.
James Fremantle, spokesman for the Air Transport Users Council, said: "We have never come across an airline doing this intentionally before.
"It is inconvenient for passengers, but is not very different to a shop closing down for refurbishment."
He said the council would support any work to make the prices more transparent.
Ryanair, which takes 90% of its bookings through the website, said its current system was close to "breaking point" and "essential system upgrade works" were needed to handle bookings.
The airline stressed that all flights over the period were operating as normal. Customers who had intended to check-in online this weekend, and were unable to do so because of the closure, would not be charged the £3 airport check-in fee.
Price transparency rules
Last August, the Office of Fair Trading ordered 13 airlines to amend their websites to include unavoidable fixed costs, such as tax, in its headline prices.
The 12 other airlines have already updated their pricing systems.
In early February, Ryanair warned that profits could be halved this year as fuel costs rise and the UK pound weakens.
The warning came as the budget airline reported that net profit dropped 27% to 35m euros ($52m; £26m) during the three months to the end of December.