I can guarantee you that at any launch meeting you attend, if someone's presenting a title due out in the spring that has even the slightest connection to environmentalism/green living/&c, the first words out of the sales rep's mouth will be: "Can we bump this to coincide with Earth Day?" Books about religious holidays (Christmas being the obvious example) come out in time for holiday shoppers; books about celebrities in the news are rushed out the door in order to capitalize on media attention; women's fiction and memoirs by female political figures come out in time for Mother's Day, and books about golf, grilling, and other assorted manly topics come out in time for Father's Day.
If you're writing literary fiction, science fiction, fantasy, or historical fiction, you might not have much in the way of "prime" timing. Be aware, however, that many more books are published in the fall than at any other time of year, so if you're pubbing in the fall, 1.) you'll have the advantage of full(er) book stores and holiday shopping madness, but 2.) so will everyone else, and you may get drowned out by the deluge of new titles.
I'm hesitant to say whether romance titles do better around Valentine's Day. My inclination is to believe that romance titles are bought year-round by very devoted audiences, so it doesn't seem likely that there'd be much of a bump for a Hallmark holiday in February. I don't think many husbands/boyfriends/partners buy many books for their wives/girlfriends/partners for Valentine's Day (which is sad!), and the only items I can think of that would see a bump are the tried-and-trues (chocolates, champagne, &c), movie tickets (hello romantic comedies!) and maybe DVDs.
If you're writing children's or YA, there's an interesting sales bump around Easter that doesn't appear in sales trends for adult titles, so you might benefit from a publication date in March or the first week of April.
Mysteries, in my experience, do well in the fall, but that could be due to the fact that everything does well in the fall (see above). I'm not sure whether Halloween exerts any influence, but I don't think it can hurt.
As previously mentioned, women's fiction tends to do well in the spring and summer, due in large part to Mother's Day and the perennial "beach read" tables in brick-and-mortar book stores. Then again, many argue that most fiction is "women's fiction," so take this with a grain of salt.
Finally, non-fiction (unless it's holiday-themed or otherwise sensitive to a particular time of year) tends to loosely follow the news cycle, so books about oil and BP did well during the Deepwater Horizon disaster, books about derivatives, mortgage-backed securities, and other financial topics did (and continue to do) well in the midst of the recession, and books about or by celebrities do well when those celebrities are in the news (everything from winning awards to getting jail time to biting the proverbial dust).
In short: absolutely ask your agent and editor about the timing of your book release, and don't be afraid to make suggestions if you think anything from Arbor Day to Yom Kippur will make an impact on your sales. Being in the right place (ideally front-of-store) is only half the battle; the other half is