I have the formula below which returns the largest sales day, but what
if the largest day covered two days, say Monday & Friday, how could I
build that in to my formula i.e. Result could be more than one value?

=IF(R197=0,"","Biggest Sales Day = "&INDEX(M188:M194,MATCH(LARGE
(S188:S194,ROW(A1)),S188:S194,0)))