Slavic Orthodox is an umbrella term for the Eastern Orthodox Churches that use Church Slavonic in their liturgies, as well as for the Slavic nations that are predominantly Eastern Orthodox (Orthodox Slavs).
All Slavic Churches, with the exception of the Bulgarian Church, use the Julian calendar exclusively, and all use it to calculate the date Pascha (Easter) is celebrated.
States
Slavic states with Orthodox majority or plurality:
Other Slavic states with minority Orthodox populations include Croatia (4.44%, 2011 census), Slovakia (0.9%, 2011), Poland (0.7%, 2011), and the Czech Republic.
References
Slavic Orthodox Wikipedia(Text) CC BY-SA