'It’s like being a kid in a sweet shop, but some of the best sweets are too high for me to reach'

Next month, Milligan, the renowned jazz pianist and improviser, returns to the venue as musical director of Celtic Connections’ showpiece 25th anniversary concert. The January 18 show, which to no-one’s surprise has sold out in double-quick time, will feature a stellar line-up including Eddi Reader, Cherish the Ladies, Sharon Shannon, Louis Abbott and Kris Drever,

In the years since it began this annual celebration of Americana, folk and world music has grown to become the world’s largest winter music festival. It has proved popular with audiences – last year’s total attendance was in excess of 110,000 - while the 2016 programme resulted in a net direct cash income of £3,899,433 for Glasgow and £1,031,099 for Scotland, according to a biannual economic impact study.

Celtic Connections has had a formidable roll-call of top musicians over the last quarter-century – Van Morrison, Joan Baez, Tom Paxton, kd Lang, Sly and Robbie, Richie Havens, Branford Marsalis, Steve Earle, Richard Thompson, Kathryn Williams, Bobby Womack, Suzanne Vega and guitarist Tommy Emmanuel, amongst countless others, alongside artistic collaborations and numerous special strands. The annual Transatlantic Sessions has itself become hugely popular, to the extent that it made its very first tour of the US earlier this year.

“It has been a pretty impressive range of acts,” acknowledges Donald Shaw, the festival’s artistic director. “Our starting-point [when approaching acts] is excellence, but we’re also really keen to show any links to roots music round the world. That’s a wide net we’re throwing out there. We’re always keen to show the connection between our own roots music and other countries'.

“In some cases it’s very obvious, as with the Americana scene. I knew the history of a lot of the songs going across the American Civil War and to the Appalachian mountains but what I didn’t realise at first was how much respect American musicians had, and have, towards the Scottish musical tradition. You only need to look at the output of someone like Bob Dylan to see, in the 1960s, there was a huge influence of all the great ballads and the Scottish tradition.

“The festival is a great way to shine a light on our own traditions, and sometimes the best way to do that is to invite a lot of international musicians to play over here.”

The festival has become well-established but sometimes word-of-mouth is what brings it to the attention of new artists. “Music is all about the oral tradition, about songs being passed down to other people,” says Shaw, “and that’s also how musicians work in life. They talk to each other about what they’re doing and where they’re going and why they’re doing it.

"My job has been made easier by the fact that musicians come here and generally have an amazing experience. They go home and talk about it and I’ll get messages from agents and managers saying, ‘My artist loved the festival, you should check out this or that artist'.

“In times when we’ve been trying to attract a bigger name I’ve often leaned on someone who has been here and who’s had a great experience and will pass on the word to others – they’ll say, ‘Look, it’s a great festival, you’ll have a good time, don’t worry about it'."

Celtic Connections fills the artistic void of January but it can be a double-edged sword. “I often say January’s great because it’s a quiet time in touring – but that it’s also not great, because it’s a quiet time in touring.

“Partly it’s good, because you’re not competing with other events or festivals, but you also have the job of persuading artists sometimes to travel from the other side of the world and exclusively come in for the festival. It has its merits and its challenges, the January programming.”

Next month’s programme is as diverse as ever. Attractions include Shawn Colvin, the classically-trained composer Max Richter, a Roaming Roots tribute to the late Tom Petty, an appearance by Alabama-born sisters Shelby Lynne and Allison Moorer, and a gig by Beth Orton, supported by Edinburgh’s Blue Rose Code.

The SSE Hydro on January 27 will host the GRIT Orchestra with a live performance of the late Martyn Bennett’s album, Bothy Culture, with a special appearance by the renowned Scottish stunt cyclist Danny MacAskill and the Skye outfit Niteworks as well as a performance by the All or Nothing Aerial Dance Company.

Looking through the programme and its diverse range of acts gives rise to the thought that, as the man in charge of it all, Shaw must have an enviable job. “Because I’m so passionate about music, to a certain extent it’s like being a kid in a sweet shop,” he says with a laugh. “But it’s just that some of the best sweets are too high for me to reach.

“Over the years, though, it has been a really enjoyable process. There are pressures associated with it: we’ve got a relatively small team delivering a pretty large festival [the 2018 show will have 2,100 artists in 300 events at 20 venues over 18 days] and though we do have a certain amount of support, and increase staff numbers in the run-up to the festival, we do have to make sure we have a really strong box-office to keep the festival at this level. It does have it sleepless nights, but it’s a privilege to do this job.”

Perhaps surprisingly, the festival has only three full-time staff – Shaw, festival manager Jade Hewat and associate producer Lesley Shaw (“no relation”). “The plus side to having such a small team is that we have the ability to really hone the festival. We know it so well ourselves. Having a small team means that you perfect how to go about things and you almost know how each other is thinking about things.”

One forthcoming show that summarises how far the festival has come in 25 years is the January 19 gig by Chris Stout and Catriona McKay. “The idea that you would put a fiddle player from Shetland and a clarsach [Celtic harp] player on the main stage of the Concert Hall, with people like King Creosote and the Scottish Ensemble, is an indicator of the journey of traditional music in this country over the last quarter of a century,” says Shaw.

He has watched no end of great shows and special collaborations over the years, but some remain with him still. “[Singer] Bobby McFerrin stands out. It was just that thing of a man on a stage, making an incredible noise. And there was Taraf de Haidouks, a group from Hungary, who played the Old Fruitmarket and came down off the stage and came into the crowd. That was amazing. And there have been so many amazing moments in my time.”

• Celtic Connections 25th Anniversary, Jan 18-Feb 4. Website: celticconnections.com. The Sunday Herald is the festival’s media partner.