To ensure that the extract also refreshes on the last day of the month at 18:00, you need to add a new extract refresh task to the workbook on Tableau Server. You can do this by selecting the workbook, selecting Refresh Extracts, and then choosing Schedule a Refresh. You can then select an existing schedule that matches your desired frequency and time, or create a new schedule with a custom frequency and time. This will add a new extract refresh task to the workbook, in addition to the existing one that refreshes every Monday at 10:00. You can also modify or delete the extract refresh tasks as needed. Reference:
Tableau Certified Data Analyst Exam Prep Guide, page 10, section "Publishing and Scheduling Data Refreshes" Tableau Help: Refresh Data on a Schedule Tableau Help: Manage Refresh Tasks