As the Maid of the Mist operates in the lower Niagara River, immediately below the Horseshoe Falls, the boat cannot safely navigate in the water unless the river is ice free. Historically, the opening date has been May 1st, although in some years with milder winters, it has open in Mid April.
You can plan on an opening day of May 1st with 99% confidence as you plan your spring time trips to Niagara Falls.

The best place to find the official opening day information is the Maid of the Mist web site which is linked here.  The Maid of the Mist is reputed to be the longest running tourist attraction in North America.  It has been in operation since 1846, although not continually.  The attraction took some time off during the Civil War.

The Maid of the Mist is truly a bucket list worthy attraction in Niagara Falls.  If you have just one hour to do one thing in Niagara, make certain it is the Maid of the Mist.  Until just a few years ago, the Maid served both sides of the border, but today that has changed.  The Maid of the Mist now departs from the U.S. side only.  Hornblower Cruises operates the franchise on the Canadian side of the river.  Which ride is better?  Ride both boats and you tell me.  The real star is the amazing Horseshoe Falls.