Google Document Spreadsheet 曜日 weekday

=IF(a2="", "", IF(WEEKDAY(a2, 2)=1, "月", IF(WEEKDAY(a2, 2)=2, "火", IF(WEEKDAY(a2, 2)=3, "水", IF(WEEKDAY(a2, 2)=4, "木", IF(WEEKDAY(a2, 2)=5, "金", IF(WEEKDAY(a2, 2)=6, "土", IF(WEEKDAY(a2, 2)=7, "日"))))))))