Does this month start on a Monday?
Excel,  44  34 23 19 18 bytes
=2=MOD(A3&amp;&quot;-&quot;&amp;A2,7

-11 thanks to Adam and -4 thanks to Shazback
-1 thanks to this
Original Answer
=(TEXT(DATE(A2,A3,1),&quot;ddd&quot;)=&quot;Mon&quot;)

Ensure that the year is placed in cell A2 and the month in A3.
:( Crossed out 44 is still 44.