I had the same question before going to my first baby fair (Aug) in my first trimester.
Yes, I notice it’s every quarter. I went to another one in Oct and planning to go next week.
Just take your time to do research and slowly buy since your EDD is June. Don’t be pressured into buying anything. Breast pumps /baby items are not cheap plus cannot try.
I tried asking about the frequency of these baby fairs / what essential items to buy / 2nd hand items to get in those FB groups but the comment functionality to my post was disabled (probably by the group admin). I guess that some of those mummy groups are maintained by the baby fair organisers or affiliated to. 😬