There are two ways to reach the Christmas season. One of which is to wait until Christmas in real life, at which point the season will activate in the game. The faster solution is to purchase the Season Switcher heavenly upgrade after one of your ascensions. Once you have the Season Switcher, switch to the Christmas season and look at your grandmas. Some of the them will now have Christmas themed outfits. Click on one of them, and you will get the achievement.