Folk-lore says garlic should be planted on the shortest day and harvested on the longest day - which is the twenty-first of December and June respectively. But really, anything from November onwards seems to work. But that relies on being organised enough to have your garlic ordered and delivered (or bought in a shop/garden centre) in time. This season I failed on this and only got my garlic in mid December. Then found preparation for Christmas scupered the chance to get a spot ready to plant it.
So, here I am, planting my garlic as the first task of the new year.