If there is a class meeting and associated lesson plan that you need to run again, you can use the Repeat Lesson Plan feature:

Class meetings for repeated lesson plans will be recorded as long as the Record Classes setting is selected on the section edit page. View this page by navigating to the desired section in Forum, clicking the Edit Section button on the right and checking the below setting.

Make sure that the session numbering is correct in Course Builder. You can do this by going to the course page in Course Builder and looking at the Syllabus Outline to ensure that:

If the session ordering looks like it's correct in Course Builder, try also republishing the lesson plans for the sessions that appear on the wrong day on the Forum section calendar.

Finally, you can also try deleting and re-adding the class meetings that are in the wrong order. You can do this by clicking Remove on the class meeting from the section calendar, and then re-adding the class meeting back to the same day. Deleting the class meeting won't delete the lesson plan–the lesson plan will automatically be added back to the appropriate class meeting.

If holidays are added to the Forum calendar, then any class meetings scheduled for those days will be skipped. For example, if a holiday is scheduled for the first Monday of September, then any sections that have a regular class meeting on Monday will have the class meeting for that day removed. The lesson plan that would have occurred on that day will be pushed out to the next meeting day.

Note that while class meetings will be automatically skipped, assignments can still be due on holidays. If you need to update the assignment due date, you can do so by changing the due date in either Course Builder (see this article) or Forum (see this article).