Visa first available date = 签证公告上的当前月的第一天。例如:F2B 类优先日期2008年12月22号以前申请人的当前月的第一天是2015年9月1号
The date that a visa becomes available is the first day of the month of the Department of State (DOS) Visa Bulletin, which indicates availability of a visa for that preference category