Good Evening Maureen, 

Thank you for contacting the University of Minnesota Extension. I will attach a link to the U of M Extension yard and garden website regarding lawn care: https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar

If you look at the information, our MN lawn care calendar, the cool season grasses begin growing as soon at the temperatures rise above 32 degrees in the spring. 

Attached is another link about winter injury to lawns: https://extension.umn.edu/yard-and-garden-news/what-does-winter-injury-look-your-lawn. 

Although this information does not specifically address leaving leaves on the lawn, I do not believe leaving the leaves over the winter will protect the lawn anymore than removing them. Any moisture we receive in the form of rain/sleet/snow will be absorbed by the lawn/soil until the ground freezes. If you left a layer of leaves, it may encourage vole damage or disease. 

The leaves would be excellent mulch to protect perennial plants, tree and bushes from the fluctuating winter temperatures. 

Keep warm!