Using NETWORKDAYS to find holiday date clashes

=SUMPRODUCT((Z1:Z10>=A2)*(Z1:Z10<=B2)) works because TRUE is 1 and FALSE is 0.

Thus (Z1:Z10>=A2) gives an array of ten values which can be viewed either as TRUE or FALSE or as 1 or 0.
similarly for (Z1:Z10>=B2)

Now 1*1=1, 1*0=0, 0*1=0, 0*0=0.
So you only get a 1 when (Z1:Z10>=A2) AND (Z1:Z10>=B2) everywhere else you get 0.

Add them up and there's your answer.

PS
=SUMPRODUCT((Z1:Z10>=A2),(Z1:Z10<=B2))
should also work and can be readily extended :
=SUMPRODUCT((Z1:Z10>=A2),(Z1:Z10<=B2),(Z1:Z10<=C2)...

Mark.