Short answers, pulled from the story.
Yaroslavl was founded by Yaroslav the Wise, a prince of Kievan Rus', during his rule over the Principality of Rostov, which lasted from 988 to 1010. He chose a promontory called Strelka, at the confluence of the Volga, Kotorosl, and Medveditsa rivers, as the site for the first Yaroslavl Kremlin.
Polish-Lithuanian forces occupied Moscow during the Time of Troubles, making normal governance impossible. From April to June 1612, the army of Kuzma Minin and Prince Dmitry Pozharsky was stationed in Yaroslavl, and the most important matters of Russian state were decided there during that period, making the city the de facto capital.
Yaroslavl's historic city center was inscribed on the UNESCO World Heritage list in July 2005. The designation recognized the area as a unique example of combining Western European and Russian imperial architectural styles, and as an outstanding example of urban planning shaped by Catherine the Great's Municipal Planning Reform of 1763-1830.
On the 7th of September 2011, most of the players on Lokomotiv Yaroslavl, the city's KHL ice hockey team, perished when their aircraft crashed on takeoff from Tunoshna Airport. The team had been flying to their first match of the season. As a tribute, Yaroslavl co-hosted the 2012 Canada-Russia Challenge with Halifax, Nova Scotia.
The Transfiguration Cathedral of the Spaso-Preobrazhensky Monastery, completed between 1506 and 1516, is the oldest unchanged detached building in Yaroslavl. The monastery itself was founded in the 12th century, but the current cathedral was rebuilt after the original structure was destroyed in 1501.
The Volkov Theater, founded by Fyodor Volkov in 1750, is recognized as the oldest theater in Russia. It has been housed in a neoclassical building in the old town since 1911, and has around 1,000 seats across two stages. It is considered one of the most prestigious playhouses in the Russian theater community.