How critical is this- I just bought the car for daugter and the light (yellow) came on yesterday service engine soon, I am taking to the mechanic on tues. should we be concerned?

I heard that the service engine light automatically comes on every so many thousands of miles. If the car doesn't seem to be having any problems then it is just a maintenance up keep reminder and not really anything to be concerned about.

Nissan doesn't list a maintenance lamp for this model.

You stated that the lamp says: "Service Engine Soon"

That SES lamp indicates that something that monitors vehicle emissions has failed, or is at least worn or on the verge of failure, and it's allowing excessive pollution.
Pollution = unburned fuel = you aren't getting the most out of your far too expensive gas.

Having said that, without drivability symptoms, you may simply have a faulty
EGR valve. A faulty EGR valve or EGR system will allow emissions to increase, as calculated by the Powertrain Control Module (PCM).

Have someone check the code and let us know.