If have high cholesterol, is eating fast food once a month bad?

Missing data: "High Cholesterol" is a rather vague description. It can be mild with little importance or one of many inherited disorders of metabolism with severe implications. You get the most from this site when you provide detailed background information. In the absence of a better history, present health & the actual numbers a proper focused answer is not possible.

How high?: It would have helped to know your actual results and other risk factors for heart disease. Eating fast food once a month, may not make a difference, but you need to evaluate your whole life style and exercise. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/. Get HPV vaccine