Yes, it is lawful--the law does not require any specific advance notice of unscheduled meetings or overtime, or even that "reasonable" notice be provided. Furthermore, a company could terminate an employee who cannot comply with unreasonable, or even frankly impossible, requests, so long as the company does not--1) breach any employment contract with the employee, or any union agreement;2) discriminate against employees due to a protected characteristic, such as race, sex, religion, age over 40, or disability; or3) retaliate against employees for having used a protected benefit (e.g. FMLA leave) or having filed a protected claim (e.g. for discrimination or overtime).