Lobster, steamed Saturday afternoon, eaten Monday night?
According to stilltasty.com two days is the max, so you'll be fine as long as you refrigerated it within two hours of cooking.

Other sources claim durations of 4-5 days. As sarge suggests, use your nose.

Other sources:


http://www.lobsterhelp.com/lobster-facts.html
http://www.fao.org/wairdocs/tan/x5887e/x5887e01.htm