Sinky, it is at least a discipline.
One could also argue that insofar as it is a VERY long-standing tradition, affirmed in writing around 300 AD, that it has the force of permanent law.
Seems that many of the posters on this thread have forgotten about Tradition, which is fully ONE-HALF of the 'Scripture and Tradition' foundation of Church teaching and law.
Law? It's a discipline. If it were a law, there wouldn't be any exceptions to it, would there?