Going off schedule
pills have no hormones in them, so it's ok to skip them. However, a pack of pills is for 4 weeks, not a calendar month.
There are 13 packs used each year, not 12. If a woman can stay on schedule even if she skips the placebo pills, then it would be ok for her to skip them. For most women, it is easier to take one pill a day and not think about when to start the next pack of pills.