If the employees are salaried (paid an annual salary in weekly or biweekly installments, with the amount not based on the hours worked), then the employer doesn't have to pay for weekend work: the salary is the total pay for all work done, no matter when.If the employees are hourly, however, they must be paid for weekend work, including overtime as applicable. Hourly employees must be paid for all work, means for anything (including events) which their employer requires them to do (if the employer makes you do it, it is "work"). There is no exception for non-profits: they have to pay when their hourly employees work the same that a for-profit company does.