Minnesota being in the Interior of the USA experiences all extremes of weather and temperatures.
Winter days can be extremely cold, so dress accordingly. Hand and toe warmers are a good and inexpensive investment. Exposed skin can freeze in minutes here, so don't turn up in shorts in January, otherwise, we will have to detour to a Outfitters store rather than looking at cool birds!
Summers are hot and quite sticky, oh and don't forget we get mosquitoes here, so sunscreen, water bottle, bug spray will make your day a much more enjoyable experience.
Spring and Fall (migration time) which is a popular time to come are generally quite pleasant, but don't be surprised if there is an odd tornado/storm warning, freeze/frost warning, or windy weather by Lake Superior. Basically, the weather can be unpredictable. Check the 10 day forecast before you leave. 10 Day Forecast for Duluth.