It does not appear that your MSA provides a clear answer to the problem. As such, there are a variety of ways to resolve the issue.
If you have the holiday this year while the he has it next year, you could agree now on how to set up the times for pick up and drop off. Whatever you have this year, he will have next year.
Another option would be to defer to other holidays. In other words, whatever you do for those holidays, you would do here as well. For example, if you consider the Thanksgiving holiday to begin with the end of school, then you would do the same here.
Another option would be to define the holiday literally. As the holiday begins at sundown, that would be when the visitation period would begin as well.
If all else fails, you could file a motion to clarify with the court. The motion would describe what you have detailed here and ask the court to more clearly define the visitation period.
Also, several customers have asked how they may direct a question to me in particular. If you specifically want me to assist you in your legal matter, just put "FOR JOSEPH" in the subject line and I will gladly pick up the question as soon as I am on-line.