Summing all transactions that are on a Friday

Re: basic SUMIF - no, not with your existing data (dates in C)

SUMIF, unlike SUMPRODUCT / Arrays, can not alter the underlying values when calculating which is what you're trying to do with TEXT.

If you want to use SUMIF you would need to either modify C such that they included or returned the weekday literally or store the ddd in another column.

Example of the former: if C7:C9 stored "ddd" (even partially) then you could use SUMIF, eg

C7: =TEXT(appropriate date value,"ddd, dd-mmm-yy")
etc

then

=SUMIF(C7:C9,"Fri*",D7:D9)

SUMIF won't let you manipulate the criteria range that way.. so you have to use Sumproduct()

=Sumproduct(--(TEXT(C7:C9,"ddd")="Fri"),D7:D9)