This annual, four-day event, set up to bring live music to the Western Isles and help preserve and develop the "rich culture of Celtic music", attracted over 17,000 people last year.
Although it is billed as the "biggest homecoming party of the year", scores of festival-goers have no family connections with Lewis but come from all over the world to sample the music and atmosphere. Around 5,000 are expected to fill the giant blue marquee in the grounds of Lews Castle to hear The Proclaimers and other musicians from the UK and abroad.
Tomorrow evening, the emphasis is on family fun, when the "The Big Busk!" kicks off at 5pm, offering free entertainment around the town from the Dream State Circus and street theatre.
During the day, festival-goers of all ages can take part in Gaelic- language taster sessions, as well as fiddle and whistle workshops, at An Lanntair, the new arts centre.
Running in conjunction with the music festival is "Sail Hebrides", a maritime festival set up to encourage young people to try sailing. Events include dinghy tuition and rowing challenges in Stornoway harbour.
An exhibition on the maritime history of Stornoway will run at the town hall until August. Celebrating Stornoway covers all aspects of the industry, including fishing boats and trade, the impact of war and the role of women.
The exhibition is supported by a programme of talks, guided walks, music, singing and storytelling.