The Employment-Based First Preference category is current for all countries.
The Employment-Based Second Preference category is current for all countries, except for China-mainland born (cut-off date: Jan. 1, 2006) and India (cut-off date: April 1, 2004).
The Employment-Based Third Preference category have the following cut-off dates:
All Chargeability Areas Except those listed (e.g. Canada): Aug. 1, 2002.
China - mainland born: Sept. 1, 2001.
India: April 22, 2001.
Mexico: April 22, 2001.
Philippines: Aug. 1, 2002.
For the Other Worker category, the cut off is Oct. 1, 2001.
The Employment-Based Fourth and Fifth Preference categories are current for all countries.
Read about the numerical limitations for immigrant visas.