An employer can set the conditions of the workplace much as it sees fit. Accordingly, it is not bound by the time contraints that a worker may have due to outside employment. This includes scheduling a meeting on an employee's day off and then disciplining them for missing the meeting, no matter the reason. This is true so long as there is no union agreement or employment contract that provides otherwise. As for treating other employees who missed the meeting differently, this too is legal unless the reason for your differing treatment was due to some form of legally actionable discrimination (i.e. was due to your race, religion, national origin, disability, age (over 40), etc.).