What coverage do you have if you buy a vehicle on the weekend?

Generally, the insurance that you had on the prior car will remain in force for a stated period of time--certainly long enough to bridge the weekend. You should, on the following business day, contact the insurance agent and advise of the replacement vehicle.

Note that if the prior car was paid off and if you dropped collision/comprehensive coverage, you will likely be required to maintain those coverages if you financed the new car.