Snow Radish Vacation is the prequel to Summer Radish Vacation 1 and 2 and as such, also of School Days. Chronologically, it is the first in the "Radish continuity."

[From a post by Arinoch on Hongfire]

At the end of the year, in a snowy region...
There's a drive-in restaurant along a no longer used highway.
The name of the restaurant is Snow Radish.
It's managed by a mother and daughter, who are quite well known in the neighborhood. The daughter's name is Kagura, and she attracts customers with her pretty face. The mother's name is Moegi.
Now, to the matter at hand... Over winter vacation, Moegi's sister and her friends come to stay at the Snow Radish.
But one of the people Moegi's sister brings along is the cram school teacher Kagura used to have a crush on.
And so a busy, fun, but slightly sad, winter break begins.

[Translated from DLSite]