Light festivals
Blackpool Illuminations is the original and the biggest of its kind in the UK, running along roughly six miles of seafront from early September through to early January. It's free to view and works well as a day trip or overnight stay, especially combined with Blackpool's other attractions.
Blackpool's Lightpool Festival runs a shorter, more concentrated window in October, focused on 3D projection and light-art installations rather than the traditional illuminated tableaux — a good complementary visit if you're already in the area for the main Illuminations.
Christmas markets and winter festivals
London's Hyde Park Winter Wonderland and Edinburgh's Christmas are the two biggest UK winter market festivals, both combining free-to-browse markets with paid rides, ice rinks and ticketed attractions. Both run from mid-to-late November through to early January.
If you're visiting either on a weekend in December, expect large crowds — a weekday visit, ideally in the first couple of weeks after opening, tends to be considerably calmer.
Planning tips
Most festival-style events on this list are free to enter, with individual rides, ice rinks or ticketed shows charged separately — budget for those extras rather than assuming the headline event is entirely free.
Booking specific timed attractions in advance is worth it for anything you don't want to miss, particularly around weekends and the school holiday windows either side of Christmas.