Silver Helper publishes free reference pages for planning around working days: how many business days a month or year contains, which public holidays reduce them, and how each number is calculated. Every count is derived from the tables on the page, so nothing has to be taken on trust.
Monday to Friday, excluding public holidays that apply to the country as a whole. Regional, sector-specific and employer-specific days are listed on the pages but not subtracted, so the counts are reproducible and you can adjust them downwards for your own situation.