As a knitter who ASPIRES to be able to make a beautiful Fair Isle jumper (but can't yet - handknitted stranded colour work is tricky, takes a huge amount of time, and needs experience in order not to end up with a puckered mess) I absolutely reject the idea that a specific traditional knitting style is 'just for Christmas'. Lots of Icelandic (think Sarah Lund-type jumpers with a patterned yoke) and north European stranded colour work knits do feature traditional designs of stylised fir trees and snowflakes, that's true - and I STILL don't think those jumpers should just be for Christmas. Those things are wintry.
Technically, only designs made with Shetland wool (Jamieson's is the gold standard https://www.jamiesonsofshetland.co.uk ) and using 2 colours per row of stitches, carrying the colour you're not knitting with on the wrong side as a 'float', and utilising traditional Fair Isle/Shetland motifs, can be called Fair Isle knits. The Shetlanders are very particular about this! See here, an impassioned open letter criticising the fatuous Ch4 show Game of Wool for daring to call their first challenge a 'Fair Isle' challenge! 
That letter went viral, and rightly so. https://www.shetlandtimes.co.uk/news/letter-you-ve-misrepresented-an-entire-culture-open-let-418907/
Only if your 'fair isle' knit is 100% squeaky acrylic and features rows of novelty motifs like stockings, Santas or candy canes would I suggest it's a December-only knit.
If you want a beautiful, timeless look (which is also pleasingly retro, depending on how you style it - I love a colour work cardigan over a tea dress) then look for second hand Brora on eBay and Vinted. (or buy it new if you have bottomless funds.)