Officially in the US, no. The MR program specifies breakfast for Platinums only on Monday-Friday.

Some properties (see the sticky threads) will open their lounges on weekends and some properties may give a breakfast voucher on weekends but it is totally dependent upon the kindness of the individual property (or front desk clerk). Asking nicely doesn't hurt...and, yes, I've gotten a couple of breakfasts that way but most of the time I'm told, "Sorry, sir..."