Bus from Bern to Krapinske Toplice(Spa) is running year around, the bus goes via Zurich and Zagreb, where passengers might have to change bus. During summer season the Thermal Spa is very well visited which is also is reflected in the number of passengers traveling on the route, so make sure to book your ticket in do time, if you travel back to Bern after you have visited the Thermal Spa, you should buy a return ticket, timetable for returning buses you can see here.