If USCIS determines that there are more immigrant visas available for a fiscal year than there are known applicants for such visas, USCIS will state on this page that you may use the Dates for Filing Visa Applications chart.
USCIS policy memo indicating that the listing of the advance notification of Diversity Visa (DV) availability in the Visa Bulletin enables the filing of DV adjustment applications with DV numbers ranked below the specified cut-off number, even though a visa is not yet available.