We have joint custody with me being the primary custodial parent. The kid's mom doesn't always come get the kids on her weekend. Then she'll appear on a weekend that is not hers and want them. Suggestions, checklists and worksheets for creating win-win parenting agreements. This may not be what is best, however.
Many experts agree that children benefit from maximum possible contact with both parents.
So, it may be best for your child for you to be flexible regarding the weekend visits. Of course, if the visitation confusion happens often, the sheer inconvenience and uncertainty may require you to stand on your legal rights. But again, this may not be what is best. If so, you may need to pony up the resources necessary to go back into court and ask for a modification, which may trigger a custody evaluation by the court. When there appears to be a conflict between what is legal and what is best, the best approach is to work toward, well, what is best.