The date that determines whether a preference immigrant visa applicant can be scheduled for an immigrant visa interview in any given month. The cut-off date is the priority date of the first applicant who could not get a visa interview for a given month. Applicants with a priority date earlier than the cut-off date can be scheduled. However, if your priority date is on or later than the cut-off date, you will need to wait longer, until your priority date is reached (becomes current). To find out whether a preference case is current, see the Visa Bulletin or telephone (202) 663-1541.
Source: U.S. Department of State.