For Example:
A Landlord issues their Tenant a 3-day notice to pay or quit on the 1st of the month that lands on a Thursday. If the Landlord delivers the notice in-person, the last day for the Tenant to pay would be the 6th of that month.
Friday the 2nd counts as the first day, Saturday the 3rd and Sunday the 4th are excluded, Monday the 5th counts as the second day, and Tuesday the 6th would be the final day.