Here in Moncton, New Brunswick, we are known to have snow days, and not just one or two. It is completely natural to have at least five or six snow days per year. Since teachers anticipate that there will be snow days, they always plan ahead so the will be able to fix the entire curriculum into the school year. But, if those are able to recall the beginning months of 2016 as when we were pounded with one blizzard after another, those are the snow days that cause genuine problems for teachers trying to complete a curriculum.
A course is made up of a curriculum and each curriculum contains units that have to be taught. Some are more detailed than others, and as Mr. Allerston showed me in regards to the Chemistry 120 curriculum, there are even page numbers for each day and how many hours that should be taught in each section of the units. Content courses like Law and Political Science are easier to divide amongst the school days, while Math courses are harder to do.
In the case of a year with many snow days, teachers find that they do not have as much time to work in class and students may find themselves assigned with extra homework or going through a unit quicker than usual.
A big pressure that teachers are likely to feel is if they have to change their pace of teaching and what parts of the curriculum they may need to focus less on in favour of the more important topics to insure that students learn the essentials of the course they are taking, while extra and unimportant topics may not have as much time to be focused on.