Lydia Emilie Florentine Jannsen, (24 December 1843 – 11 August 1886), known after her pen name Lydia Koidula was an Estonian poet. Her sobriquet means 'Lydia of the Dawn' in Estonian. It was given to her by the writer Carl Robert Jakobson. She is also frequently referred to as Koidulaulik – 'Singer of the Dawn'.
In Estonia, like elsewhere in Europe, writing was not considered a suitable career for a respectable young lady in the mid-nineteenth-century. Koidula's poetry and her newspaper work for her populist father, Johann Voldemar Jannsen (1819–1890) remained anonymous. In spite of this, she was a major literary figure, the founder of Estonian theatre, and closely allied to Carl Robert Jakobson (1841-1882), the influential radical and Friedrich Reinhold Kreutzwald (1803-1882), writer of the Estonian national epic, Kalevipoeg (The Son of Kalev).
Lydia Jannsen was born in Vändra, Pärnu County, Governorate of Livonia (now in central Estonia). The family moved to the nearby county town of Pärnu in 1850 where, in 1857, her father started the first local Estonian language newspaper and where Lydia attended the German grammar school. The Jannsens moved to the university town of Tartu, the most progressive town in Estonia, in 1864. Nationalism, including publication in indigenous languages, was a very touchy subject in the Russian Empire but the rule of Czar Alexander II (1855-1881) was relatively liberal and Jannsen managed to persuade the imperial censorship to allow him to publish the first national Estonian language newspaper in 1864. Both the Pärnu local and the national newspaper were called Postimees (The Courier). Lydia wrote for her father on both papers besides publishing her own work. In 1873 she married Eduard Michelson, a Latvian army physician, and moved to Kronstadt, the headquarters of the Russian navy near St. Petersburg. In 1876-78 the Michelsons visited Breslau, Strasbourg and Vienna. Koidula lived in Kronstadt for 13 years but despite spending her summers in Estonia, she never stopped feeling inconsolably homesick. Lydia Koidula was the mother of three children. She died on August 11, 1886 after a long and painful illness. Her last poem was Enne surma- Eestimaale! (Before Death, To Estonia!).