When is the best time to take a break?

Contrary to the common misconception, freelancers are not always on vacation! True, many freelancers choose to take the leap into self-employment to escape the daily grind of the corporate structure and take control of their lives. Certainly, there is no doubt that freelancers benefit from a healthier work/life balance, however, this only applies if they actually stop working from time to time! The downside is that freelancers don't have set holiday time and often fear that taking a break will result in missed opportunities or jeopardise an existing client. 
This got me thinking. Are there patterns to a freelancer's schedule? Are there certain times in the year when taking a holiday makes more sense than staring at a screen all day? Do freelancers have slow months? If so, when are they and could this help add a bit of structure to an otherwise relentless workload? 
The big caveat to the question is that it depends greatly on the type of freelancer you are and the sector you specialise in.
I'm a freelance writer and have always found August and December to be the quietest times of the year. Logic would suggest that this is due to my clients and prospective clients flocking to sunny climates in the summer and slowing down for Christmas in the winter. 
The kicker is that if I choose to take a holiday when they do, I have to join the rat race but, hey, at least I get to have a break!