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|999 by topic|
|Ab urbe condita||1752|
|Balinese saka calendar||920–921|
|Chinese calendar||戊戌年 (Earth Dog)
3695 or 3635
— to —
己亥年 (Earth Pig)
3696 or 3636
|- Vikram Samvat||1055–1056|
|- Shaka Samvat||920–921|
|- Kali Yuga||4099–4100|
|Japanese calendar||Chōtoku 5 / Chōhō 1
|Minguo calendar||913 before ROC
|Seleucid era||1310/1311 AG|
|Thai solar calendar||1541–1542|
1125 or 744 or −28
— to —
1126 or 745 or −27
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Year 999 (CMXCIX) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. Depending on how one defines a millennium, 999 was either the last or second-to-last year of the first millennium.
- The Liao Dynasty Emperor Shengzong commences annual attacks, against the Song Dynasty of China.
- The Samanid Dynasty ends with the invasion of the Karakhanids, from north of the river Syr-Darya.
- The Marth Mariam Forane Church is established in Arakuzha Village, in Kerala.
- December 30 - The Battle of Glenmama is fought in Ireland.
- Silesia is incorporated into territory ruled by Boleslaus I of Poland.
- The Orsay Commune is founded in France.
- April 2 – Gerbert of Aurillac becomes Pope Silvester II, and succeeds Pope Gregory V as the 139th pope.
- Sigmundur Brestisson introduces Christianity in the Faroe Islands.
- Christianity is adopted in Iceland.
- February 7 – Boleslaus II the Pious, Duke of Bohemia
- February 18 – Pope Gregory V (b. c. 972)
- June 11 – Ebergar, Archbishop of Cologne
- December 16 – Saint Adelaide of Italy, German empress and regent (b. 931)
- December – Elisabeth of Vendôme, countess, wife of Fulk III of Anjou (aka Fulk the Black)
- Maredudd ab Owain, King of Gwynedd
- Subh of Cordoba, mother and regent of the Caliph of Cordoba (b. 940) (approximate date)