The best time to buy a particular model is when it is ON SALE. That might be the last week of December ... Or not.
Typically dealers are willing to make the best deal right before sales support for that model and year ends. Once BMWFS ends sales support (low APR, subvented leases, OLP, option credits, etc.) its harder for them to find a buyer. Of course, that means buying a prior model year off the lot, and it may not have the colors and options you want.
Example: in September of 2011 there were some KILLER deals on leftover 5 GT launch cars. The two I looked at both had HUD but no NAV. Easy to see why they sat on the lot forever.
Sometimes BMWUSA is neck and neck with Audi and Mercedes, trying to be the #1 seller of German luxury cars. In those years (and 2012 may or may not be one of them), BMW corporate heavily subsidizes sales to SIB the sales crown.
One thing I've learned after 8 BMW's. There is ALWAYS some model on sale.
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2014 535i MSport
Space / Black, ZCW, ZDA, ZLP, ZLS, ZPP, H-K and 704
2014 328i Sport Wagon
, Glacier / Black, ZSL, ZD2, ZD3, ZDH, ZLP, ZPP, ZTP, and H-K
2011 535ix MSport
2011 X5 35D
2008 ///M3 Vert
2008 X5 3.0
2007 X5 3.0
2006 X5 3.0
2006 550iA SP
2003 540iA M-Technic