So, =MOD(DATE(2012,6,1)-DATE(YEAR($A$1),MONTH($A$1),C$2),21)=0
could be
=AND(MOD(DATE(2012,6,1)-DATE(YEAR($A$1),MONTH($A$1),C$2),21)=0,LEFT(TEXT(C$2,"ddd"),1)<>"s")
or you could use:
=AND(COUNTIF(B$4:B$18,B4)=1,NOT(ISERROR(MATCH(B4,B$19:B$30,0))),LEFT(TEXT(C$2,"ddd"),1)<>"s")