1. Look at how people pay for these services. If a lot of people use insurance, revenue will be pretty evenly distributed throughout the year perhaps. That depends on how insurance reimburses doctors
2. If more pay out of pocket, I'd look at when consumer spending is seasonally low. That's likely post-Christmas. Also look into when people tend to get braces and such, do a lot of people try and do this in summer when school is out?
3. I bet a lot of people finance braces and so forth. That means they'll likely pay whoever is financing over time. If you self-finance (unlikely), they'll pay you over time. If you hire a 3rd party, they'll likely pay you upfront and then they'll get the monthly payments.