I agree with Parth that planning ahead is important to avoid this type of pitfall. During these weekly meetings, they should set a goal for what should be done and make sure they meet it. So if a person mentions they will be going on vacation or if there is a holiday, during the weekly meeting they can discuss what work they need to get done before they go. You should have a conservative plan and an aggressive plan as well. This will allow you to have a more realistic goal as well as what more can be done if you speed up certain processes.