August + September (and, to a smaller extent, December + January) are HUGE in bookselling. For textbooks, that is. And that includes fiction and nonfiction trade books that get used as textbooks, too. But that's B2C only; a month or two earlier is where all the B2B purchasing is happening, and a large hunk of that is used textbooks...which the author/publisher makes no money on since it's the secondary market and all that.
Buying books for kids for summer reading is a thing. And there's "gift books" like Oh! The Places You'll Go! and whatever else you buy graduates. There's beach reads/books to read on the plane, so that would be summer (exact months depending on the hemisphere we're talking about). You could probably map out the dates that the most births happen and then add/subtract however many months for your pregnancy/child birth/child rearing books.
There's also "Macroeconomic Conditions
" that cause trends, too. Like gardening, housekeeping, home improvement, cooking, and spirituality + self help books got huge when covid started. And Stephen King's The Stand lol. And books like How To Be Antiracist pop up the sales chart every time something in the news happens that reminds people that racism is a thing that still exists. And every time a celebrity dies, all of their books (or books about them) sell out. Or Tucker Carlson decides that a book is going to be burned by the radical left, so buy the Cat in the Hat while you still have your chance. Or whoever is the leftist version of Tucker Carlson decides that a book is going to be burned by the radical right, so buy And Tango Makes Three while you still have your chance.
This is all a very long way of saying: Your question is incredibly vague and no one can give you good advice until we know what your book is/is about and who your target audience is. And even THEN we cannot see the future, so who knows what thing will happen that will make your book suddenly super popular and/or relevant. So you need to always have your thinking cap on and paying attention to the world around you and take advantage of any chances that spring up.