People ask when to book an alpine winter. The honest answer depends on one thing. Are you tied to a particular week, or to a particular place?
If you need the week between Christmas and New Year, you are competing with families who have held the same chalet for a decade. Ask in spring for the following winter and you are early enough. Ask in autumn and you are choosing from what is left.
If the dates are flexible, almost everything opens up. January is quieter and colder and the skiing is better. March has long light and terraces. Both are easier to arrange, and both are more pleasant.
The rest is logistics: transfers that account for weather, ski school places, restaurant tables that fill differently in a resort than in a city. We handle those together rather than separately, which is the only reason it feels simple from your side.
Two of the days in this piece exist as arrangements in their own right. Jungfraujoch is the one worth booking ahead for weather rather than for space, and Zermatt is the one we will argue should be more than a day.