Thousands of undocumented immigrants have until Saturday to apply under a 1986 program that may never be repeated. Almost 20 years have passed since Congress approved an amnesty for nearly 3 million immigrants living illegally in the United States, but for perhaps as many as 100,000 undocumented residents, the door is still open. It closes Saturday. With the final deadline approaching, lawyers are reaching out to those immigrants who may still be eligible for green cards under the 1986 amnesty program.