​It is very very very very rare.
It happened twice, in 1929 and in 2012 (there were lots of talking about this coincidence: both are years of a great economic crisis).

Anyway, as soon as it happens some big boats equipped to break the ice go through the canals and slowly break it.
In 1929 actually, there were none of these boats so people could just wait until it melts. It was very unusual and people were walking all over the ice everywhere.
This is what newspapers say about the event.
Unluckily, I wasn't even born to witness it :)