While there's no doubt some books are seasonal in their appeal and need to be launched and promoted accordingly, many other books are seasonless. Therefore, if you've written a book and are planning its launch, should you avoid some months. Opinions will vary depending on which book publicist you ask, but experience shows successful book launches happen throughout the year. For example, a lot of people automatically think they should avoid the November-December holiday period. But book promotion procs routinely feel otherwise, and some say the effect of the holidays is overblown.
If you like to go against the flow and can see its advantages, launching a book in November or December means you face much less competition for the media's attention. Publicists promoting books late in the year have found more accessible openings for major shows and articles because others haven't spoken for them. The media stays open for business 365 days a year and has pages and airtime to fill. When you're available with an interesting topic at a time of year when others aren't competing as hard for their attention, you may do quite well. If you're launching a book, it's something to consider.
You can think of summer similarly to the holidays: less competition in the book promotion marketplace. News organizations produce content year-round, and they need material in the summer just as much as they do during the spring and fall, which are considered the prime book launch seasons by some. When editors and producers are scrambling to find interview guests and topics for articles, you'll do well if you make yourself and your book available. There are many perceived rules in various businesses and industries going by the wayside these days, and launch timing is one of them.
Then some books are very time-sensitive. For example, a book about a particular sport might be most successfully released when fan interest is running high at the beginning of a season. Publicity campaigns that coincide with events in the yearly calendar can do quite well when the media and readers are in the mood for information about a topic. The bottom line is that every book is unique, and publishers and authors need to do what's best. But there is more competition and fewer limits than ever before, and so you might plan something that only a few years ago would never have been done.