Max wrote...
One way

Try, array-entered:
=IF(ISERROR(AVERAGE(IF(('Summary Days'!$D$3:$D$1500="2A")
*('Summary Days'!$K$3:$K$1500=38)
*('Summary Days'!$O$3:$O$1500<0),'Summary Days'!$O$3:$O$1500))),
"",AVERAGE(IF(('Summary Days'!$D$3:$D$1500="2A")
*('Summary Days'!$K$3:$K$1500=38)
*('Summary Days'!$O$3:$O$1500<0),'Summary Days'!$O$3:$O$1500)))
....

Too inclusive. The OP's formula would only have returned #DIV/0! if
there had been no cells matching the criteria. That's easily tested
using COUNT.

=IF(COUNT(('Summary Days'!$D$3:$D$1500="2A")
*('Summary Days'!$K$3:$K$1500=38)
*ISNUMBER('Summary Days'!$O$3:$O$1500)),
AVERAGE(IF(('Summary Days'!$D$3:$D$1500="2A")
*('Summary Days'!$K$3:$K$1500=38)
'Summary Days'!$O$3:$O$1500)),"")

This has the advantage of propagating error values in these ranges, if
any, as well as not masking errors arising from nonconforming ranges.