Car dealers in Dubai are pretty good when it comes to deals and discounts. The obvious times in the year to get deals are holidays (Ramadan, Eid, xmas and so on).
That said, there are deals to be found each and every month, you just need to look. Check out: http://yallacompare.com/car-deals/ or just walk right into the the dealers showroom and see what they have.
As with any negotiation, try not to leave it until the last minute, you should take your time to consider whats best for you and negotiate as hard as you can.