Any time worked over 40 hours within a week is considered overtime. Pay periods start every Sunday and end every Saturday. Overtime pay is "time and a half" and will be included within the following week's paycheck.
Keep in mind that working an "overtime day" won't be at the overtime pay rate if you aren't over 40 hours for the week. For example, if you have worked 35 hours for the week and then work an 8-hour shift on an overtime day you would have 40 hours at your regular pay rate and 3 hours of overtime pay.