High Season: March through early June, then End of September to November.
Pros: You have a better chance of having great weather and more options for outdoor ceremonies or receptions.
Cons: Vendor availability is more scarce at these times and requires you to book those great vendors far in advance.
Off Season: December through February, then Mid-June to mid-September. (Essentially, when it is either very hot or freezing.)
Pros: Some venues offer seasonal discounts, so you can save money by getting married in our off-season. Vendors typically have more availability as well!
Cons: If you've envisioned an outdoor ceremony or reception, try to book a venue with heating and cooling options to keep you and your guests comfortable.