Answer by taking a quote from the following article:

Thomas Pynchon was born in 1937 in Glen Cove, Long Island, New York, one of three children of Thomas Ruggles Pynchon, Sr. (1907-1995) and Katherine Frances Bennett (1909-1996). His earliest American ancestor, William Pynchon, emigrated to the Massachusetts Bay Colony with the Winthrop Fleet in 1630, then became the founder of Springfield, Massachusetts in 1636, and thereafter a long line of Pynchon descendants found wealth and repute on American soil. Aspects of Pynchon's ancestry and family background have partially inspired his fiction writing, particularly in the Slothrop family histories related in the short story "The Secret Integration" (1964) and Gravity's Rainbow (1973).

A variety of rumors pertaining to the subject matter of Against the Day circulated for a number of years. Most specific of these were comments made by the former German minister of culture Michael Naumann, who stated that he assisted Pynchon in his research about "a Russian mathematician [who] studied for David Hilbert in Gottingen", and that the new novel would trace the life and loves of Sofia Kovalevskaya.  In July 2006, a new untitled novel by Pynchon was announced along with a synopsis written by Pynchon himself, which appeared on Amazon.com, it stated that the novel's action takes place between the 1893 Chicago World's Fair and the time immediately following World War I. "With a worldwide disaster looming just a few years ahead", Pynchon wrote in his book description, "it is a time of unrestrained corporate greed, false religiosity, moronic fecklessness, and evil intent in high places. No reference to the present day is intended or should be inferred." He promised cameos by Nikola Tesla, Bela Lugosi and Groucho Marx, as well as "stupid songs" and "strange sexual practices". Subsequently, the title of the new book was reported to be Against the Day and a Penguin spokesperson confirmed that the synopsis was Pynchon's.  Against the Day was released on November 21, 2006, and is 1,085 pages long in the first edition hardcover. The book was given almost no promotion by Penguin and professional book reviewers were given little time in advance to review the book. An edited version of Pynchon's synopsis was used as the jacket-flap copy and Kovalevskaya does appear, although as only one of over a hundred characters.  Composed in part of a series of interwoven pastiches of popular fiction genres from the era in which it is set, the novel inspired mixed reactions from critics and reviewers. One reviewer remarked, "It is brilliant, but it is exhaustingly brilliant." Other reviewers described Against the Day as "lengthy and rambling" and "a baggy monster of a book", while negative appraisals condemned the novel for its "silliness" or characterized its action as "fairly pointless" and remained unimpressed by its "grab bag of themes".

What did other people say about the novel?
while negative appraisals condemned the novel for its "silliness" or characterized its action as "fairly pointless" and remained unimpressed by its "grab bag of themes".