Setting of Mrs.Frisby and the Rats of Nimh

In Robert C. O'Brien's book Mrs.Frisby and the Rats of Nimh is a fun tale about a family of field mice struggling to get past moving day with a sick family member Timothy. This takes place "in an underground house in Farmer Fitzgibbons's vegetable garden" (pg.3) near the end of winter. Which is the hardest season for Mrs.Frisby and her children.