If anyone can provide more detailed dates (especially publication dates), please do so.
Also, one can find many more anniversaries just by searching on any year in Wikipedia and then clicking on Literature.

And...note the sad one near the end, under "70th."

900th anniversary
Omar Khayyam

800th anniversary
"Younger Edda" (aka "Prose Edda")

450th anniversary
June: Ben Johnson

400th anniversaries
Jan: Moliere
Apr: Henry Vaughan

350th anniversary
May: Joseph Addison

300th publication date anniversary
"Moll Flanders"

300th birth anniversary
Apr: Christopher Smart

250th anniversaries
Mar: Friedrich von Schlegel
Oct: Samuel Taylor Coleridge

200th death anniversaries
June: E.T.A. Hoffmann
July: Percy Bysshe Shelley

200th publication date anniversary
Dec 24: "A Visit from St. Nicholas"

200th birth anniversaries
Apr: Edward Everett Hale
May: Edmond de Goncourt
June: Felix Darley (American illustrator of Charles Dickens' works)
Oct: Thomas Hughes ("Tom Brown's School Days")
Dec: David Masson
Dec: Matthew Arnold

150th death anniversary
Oct: Fanny Fern

150th publication date anniversaries
"The Adventures of Tom Sawyer" by Mark Twain (rough draft only)
"Erewhon" by Samuel Butler
"The Princess and the Goblin" by George MacDonald

150th birth anniversaries
Jan: Eleanor Fortescue-Brickdale (British painter & illustrator of "Idylls of the King")
Jan: Zane Grey
May: Bertrand Russell
May: W. Heath Robinson (British cartoonist & illustrator of "Don Quixote")
June: Paul Laurence Dunbar
Aug: Aubrey Beardsley
Aug: Max Beerbohm
Oct: John Cowper Powys
Nov: Leonara Speyer
Dec: Johan Huizinga
Dec: Rene Bull
Dec: Albert Payson Terhune
Dec: Pio Baroja

120th anniversary
Dec: Tatjana Mavrina (Russian illustrator & 1976 Hans Christian Andersen Medalist; some say she was actually born in 1900)

100th death anniversaries
Jan: Nellie Bly
Nov: Marcel Proust

100th publication date anniversaries
"Babbitt"
"The Golden Bough"
"The Hairy Ape"
"Siddhartha"
"The Waste Land"
"Ulysses"
"Rootabaga Stories" by Carl Sandburg
"The Velveteen Rabbit" by Margery Williams
"The Pinafore Pocket Story Book" by Miriam Clark Potter

100th birth anniversaries
Jan: Albert Lamorisse ("The Red Balloon")
Jan: Zbigniew Rychlicki (Polish illustrator & 1982 Hans Christian Andersen Medalist)
Feb: Louis Glanzman (American illustrator of "Pippi Longstocking")
Mar: William Gaines (American publisher of MAD Magazine)
Mar: Jack Kerouac
Mar: Dick King-Smith ("The Sheep-Pig")
Apr: Bernard Evslin ("Heroes & Monsters of Greek Myth")
Apr: John Braine
Apr: Kingsley Amis
Apr: John Christopher
May: John Rowe Townsend (British author/critic: "The Intruder," 1969)
May: Hal Clement
June: Mollie Hunter (Scottish fantasy novelist & 1974 Carnegie Medalist)
Aug: Philip Larkin
Aug: Alain Robbe-Grillet
Sep: Rosa Guy (Trinidad-born American novelist: "The Friends," 1973)
Sep: Pauline Baynes (British illustrator of "Narnia Chronicles")
Sep: Mavis Gallant
Oct: Nicholas Stuart Gray (Scottish playwright: "The Other Cinderella," 1958)
Nov: Kurt Vonnegut
Nov: Charles Schulz ("Peanuts")
Dec: Grace Paley
Dec: Stan Lee ("Spider-Man")
Dec: William Gaddis
Dec: Jane Langton (American murder mystery novelist & Newbery Honoree - "The Fledgling," 1980)

70th birth anniversary
Mar: Douglas Adams

50th death anniversaries
Jan: Padraic Colum
Jan: Maxwell Armfield
Mar: Violet Trefusis
Apr: Jessie Gillespie (American illustrator of "The Birds' Christmas Carol" by Kate Douglas Wiggin)
May: Cecil Day-Lewis
Aug: Maj Lindman (Swedish writer/illustrator of series "Flicka, Dicka and Ricka" & "Snipp, Snapp and Snurr")
Nov: Ezra Pound