When you book a class, you are booking that day and time each week. Once you register, that spot is reserved for you on a weekly basis until you choose to unenroll.
When you sign up you will only be charged based on the number of classes remaining in that month. Lessons are billed as a monthly tuition, and you are always billed a month in advance. You will be charged on the 20th of the month for your classes that will occur the following month.