Hi @toimisto ,
First you would have to load all the posts sorted by the custom field (Year) as guided in this article.
https://www.advancedcustomfields.com/resources/orde-posts-by-custom-fields/
With the posts loaded, You will then need to maintain a variable to check the current year inside the loop.
If the year from the current post and the one saved in your variable is not the same, you display the new value and assign it to the variable.
Hope this helps.
