Foot passenger bookings are required for this weekend on the Stornoway – Ullapool routes says Cal Mac.
The ferry is extremely busy during the Hebridean Celtic Festival period.
Thousands of music fans are travelling to and from the islands with almost 30 international acts performing in the main arena programme, including headliners KT Tunstall, The Shires and Tide Lines – as well as events in various venues in Lewis and Harris.
Extra sailings have been arranged for Sunday to cope with the mass exodus of people the end of the festival.
Sunday’s lunchtime departure from Lewis is now fully booked for foot passengers and vehicles.
At present, some spaces are available on the 6am and 8pm sailings.
Cal Mac is urging travellers to buy tickets in advance.
The shipping firm said e-ticket customers should print their tickets at home and have tickets ready prior to boarding.
MV Loch Seaforth will operate three runs from Lewis and two sailings from Ullapool from dawn to nightfall on Sunday.
Citylink has also amended their timetable on this date to link with the ferry service. Advance reservations are necessary to guarantee a seat.
Meanwhile, HebCelt has a pop-up information and tickets office on board the ferry so people can avoid the quques at the ashore ticket base.
The ships restaurant also has a festival-themed menu.
Over the past two days, a trio of musicians from Fèis Rois, the Gaelic arts and music organisation, have been entertaining passengers.
The festival has a major economic impact on the islands and plays a key role in attracting visitors to the area.
More than half the audience comes from outside Lewis and Harris and an independent report last year showed the festival directly generated £2.1 million.