It is not “when”, but rather “where”. E. g. I live in the Northwest (Lower Saxony, near Bremen) and use “Samstag” all the time, albeit — according to studies — “Sonnabend” is prevalently used in Northern and Central Germany. I don’t hear it a lot, though, but I think I remember that our first landlady in Germany (born in the 20ies in Eastern Pomerania, today Polish territory) did use it quite often.
If you read newspapers or watch news on TV, you’ll see that the prevalence of the shorter “Samstag” is 100 % or close to that. In more relaxed contexts, you may hear and read Sonnabend, but I think it is slowly disappearing.