Given that 28 is a multiple of 7, and not of 5, I would read "28 days" (in the absence of any other information) as meaning "4 weeks", rather than "28 working days".
However if your post refers to a PPI refund (or similar) then it might well turn out to mean 'working days'. Quote (from Barcalays):
"In most cases you’ll receive the refund within 7 working days – however in some cases this can take up to 28 working days . . . "
https://www.barclays.co.uk/help/ppi-information/when-will-i-get-ppi-refund/