Bernina Express; how often do trains run?

we are planning a trip on the bernina express next year leaving from Chur to Tirano in May 2017. we are wondering how often the train runs and if we are better to travel from Chur in the afternoon and stop the night in St Moritz and continue next day to Tirano. we then have to catch a train from Tirano to Lake Como Italy and be there by afternoon 14th May 2017. Can any one help with ideas?

Chur to Tirano is every hour with change trains.

Bernina Exppress is one train a day, but same route.

See the time table.