It's the same thing every year, and every year I forget it. Then after I notice that things are slowing down I realize that this happened last year too, and it's nothing to worry about.
(I do have to add that I'm seeing a LOT of last-minute wedding cake bookings this year, for whatever reason. Dates that were totally open are being booked up three weeks ahead of time, which is pretty unusual. So maybe the cycle is changing...)
Since I basically just do wedding cakes, I deal with the cycles of the wedding seasons. There's engagement season, which is right around the end of the year. There's booking season, which is the beginning of the year and the early spring. That's when you go crazy keeping up with tastings etc.
Then comes a lull, which is usually April through June, when not as many people are booking dates. Things slow down a little then, but that's about the same time that weddings actually start to happen, so you'll be busy doing more cakes then so you won't notice.
In the late summer/early fall, bookings will start to pick up again, and weddings pick up again, but then in the late fall/early winter things slow down a lot.
If you look on google trends and put in "wedding cake" or "wedding cakes" you'll see a very clear picture of the dropoff in searches for cake toward the end of the year. This year there was an unusual spike right around the Royal Wedding, but that's obviously an anomaly!
I'm sure that the pattern is different for other places, but I guarantee that no matter where you are, there's a pattern.
One way to deal with this is to go back and look at your calendars from previous years. You'll see patterns of when you have a lot of cakes booked, when you have few cakes booked, and what type of cakes they were.
Using your calendars to track trends in how busy you are will be helpful in reminding you that you don't need to panic when you have fewer cakes scheduled at certain times of the year. If it's the same for you every year then things are going along as usual. You just need to track your own trends to see what to expect so that you can plan for the slower periods of the year.