Black Friday is a good time. This past November I picked up a ThinkStation p720 for like 60% off. Not sure whether laptops are discounted to the same extent, but it's worth investigating.

At any time of year, see if you are eligible for their "Tickets At Work" program which offers sizable discounts. They sometimes even stack with seasonal sales...

Holidays. Memorial Day next, July 4th, Labor Day, then probably not much until Black Friday sales.

Could also watch or set up relevant alerts with model numbers on Slickdeals.