Can anyone please let me know the 'typical' weather conditions in Prague
during February?
I'm guessing very cold, but not too cold to make a weekend break
worthwhile?
I haven't been in Prague in February, but I have been in Kosice (a long way
east) and in Munich and Vienna (south and west) and Berlin (north). In all
four it was bitterly cold, with ice on footpaths and lying snow, but not
unpleasant because it was dry cold and there was little wind. There were
quite a few sunny days. As long as you have plenty of warm clothing
(including boots, gloves and a hat) it should be fine.

Does anyone know if the major attractions open at that time of year?
The castle certainly is open in winter, though maybe not as long hours as in
summer. I expect that enough of the attractions will be open to keep you
busy.

HTH

Anne