The move came as Barra islanders won an emergency service for the Barrathon half marathon on Saturday if the stoppage goes ahead.
Talks between the RMT and CalMac ended without agreement yesterday and the Scottish Government said today the strike “looks highly likely at this stage”.
The government said major vessel routes to the Western Isles, Mull, Islay, Bute, Arran, Coll and Tiree were expected to be severely affected, “and only supported by smaller vessels in most cases”.
However, CalMac said the majority of its services would run during an accompanying overtime ban called by the RMT for tomorrow and Thursday.
It has published revised timetables for tomorrow and Thursday, and is drawing up timetables for Friday.
Deputy First Minister John Swinney said: “Ministers are speaking to CalMac and the RMT this afternoon to urge them to call off the action and return to talks.
“I would urge both parties to continue discussions in an effort to reach an agreement that avoids damaging strike action.”
The RMT action is about fears over pay, conditions and pensions as Scottish Government-owned CalMac faces competition for its next contract of up to eight years from private-sector rival Serco.
The RMT has demanded a written guarantee of no compulsory redundancies, and protection of pay and conditions, in its collective agreement with CalMac.
Organisers of the annual Barrathon on Saturday had called for a midnight service at the end of the 24-hour strike so hundreds of runners could reach Barra in time.
More than 300 people are expected to take part in the 13-mile race and a fun run, most of whom travel to the island.
CalMac responded today by arranging a special sailing from Oban to Barra, leaving at 12:46am on Saturday and arriving in Castlebay at 5:46am.
In a letter to CalMac managing director Martin Dorchester, Katie MacNeil, one of the organisers, had written: “I hope you are able to arrange a sailing from Oban to Castlebay as soon as the strike ends to assist the many runners in this year’s Barrathon whose travel plans will be disrupted by the industrial action on Friday.
“It is an event that draws runners to the island from across the world and raises thousands of pounds for Macmillan Cancer Support and a wide range of local charities and groups.
“If participants and spectators are unable to reach Barra in time for the Barrathon it will harm not only the Barrathon and the charities it supports but also the fragile local economy.”
Loganair, which operates flights to the Western Isles for Flybe, said it had drafted in a larger aircraft for flights to Stornoway on Friday.