Scientifically, 24 hours and the average time that the earth makes one rotation on its axis. In business, a day can be designated as a work day (eg., a Business Day) or a non-work day (eg., a weekend or a Holiday). Your locale may have different holidays than the locale of your source or your buyer. Don’t expect the work days or holidays in other countries, states, or cities to be the same as in your town.
So, during any transaction that includes a number of days specified, confirm that both parties are clear on how many days that is. If your package is expected in 10 days, are those 10 Calendar Days or 10 Business Days? Are there government or local Holidays involved that exclude a number of calendar days from the count? Ask specifically, “So, what day would you expect that to be?”