From Penny Sansevieri's book, "From Book To Bestseller":
"The two best days for your book to arrive on a reviewer's desk are Thursday and Friday."
(Earlier in the week is the backlog from the weekend and your book might be lost in the shuffle.)
"Generally, January and February tend to be the best months for submitting your book for review ... avoid sending these October & November ... The second worst months are April & May."
(A lot of big publishers are sending out their fall & spring releases at these times. Again, you might get lost in the suffle.)