According to the Vermilion Lake Association newsletter the latest date the traps were set was May 3rd. That was in 2013, the earliest was the year before March 26, 2012.
Another point of reference, Lake Zumbro went out today - the 2nd lake to go out in the state and the first without the help of an ice breaker (lol). This was twenty six days later than the median ice out date and a new record for latest ice out.