These dates are very important, as they are deadlines on your ability to opt out (or in) to LTA course materials (12th class day) and when you must pay for your LTA materials in What I Owe (20th class day).
While the actual dates vary from semester to semester, the 1st class day of the semester is the day indicated in the academic calendar on the Registrar’s web site as the day classes begin for the semester. You can count from there, including every day Monday through Friday but omitting any official university holidays (also indicated on the Registrar’s academic calendar).