But analyzing the business in question in terms of its possible effect on prospective competitors helps to explain why an activity can occur "so infrequently" as to preclude a designation as a business regularly carried on. While the operation of a parking lot on a weekly basis occurs sufficiently frequently to threaten rival parking lot owners, the hospital auxiliary's annual sandwich stand is too infrequent a business to constitute a threat to sandwich shop owners. The competition in this case is between the NCAA's program and all publications that solicit the same advertisers. The competition thus includes weekly magazines such as Sports Illustrated and other publications which solicit automobile, beverage, photocopier, and fried chicken advertisements, to name a few. Viewed in this context, we conclude that the NCAA program, which is published only once a year, should not be considered an unfair competitor for the publishers of advertising. Application of the unrelated business tax here therefore would not further the statutory purpose. We hold that the NCAA's advertising business was not regularly carried on within the meaning of the Code