29 C.F.R. § 531.57
The employee must receive more than $30 a month in tips "customarily and regularly" in the occupation in which he is engaged in order to qualify as a tipped employee under section 3(t). If it is known that he always receives more than the stipulated amount each month, as may be the case with many employees in occupations such as those of waiters, bellhops, taxicab drivers, barbers, or beauty operators, the employee will qualify and the tip credit provisions of section 3(m) may be applied. On the other hand, an employee who only occasionally or sporadically receives tips totaling more than $30 a month, such as at Christmas or New Years when customers may be more generous than usual, will not be deemed a tipped employee. The phrase "customarily and regularly" signifies a frequency which must be greater than occasional, but which may be less than constant. If an employee is in an occupation in which he normally and recurrently receives more than $30 a month in tips, he will be considered a tipped employee even though occasionally because of sickness, vacation, seasonal fluctuations or the like, he fails to receive more than $30 in tips in a particular month.
29 C.F.R. §531.57