For 2018, I'm posting a 4-part series of Slavic pagan holiday dates, one for each season. I'm posting the Winter 2018 dates in December of 2017, and I'll try to post the dates for Spring just before Spring, and so forth. These holidays are drawn from various Slavic traditions and nations. Some of these holidays are reconstructed pagan holidays from modern day reconstructionist pagan religions. Some holiday dates are currently celebrated in their countries of origin. Holidays which have been continuously celebrated from ancient times down to modern times are also celebrated by Christians. 

January

 1 Велесової бороди (Ukrainian Ridnoveri), Дідух (Ukraine)

 3 Day of Remembrance for Princess Olga (American Russian)

 4 Свято Богині Дани, Водокрес (Ukrainian Ridnoveri)

 6 Туриси (Slavic Reconstructionist), Mesojedje begins (Modern Rodnovery), Badnjak (Serbia), Бадник (Macedonia)

 8 Babin Dan (Modern Rodnovery)

 13 Malanka (Ukraine)

 17 Rijeka begins (Croatia)

 18 відунське свято (Ukrainian Ridnoveri)

 19 Koleda (Serbia)

 21 Бабинден (Bulgaria)

 24 Ptaškowa swajźba begins (Sorbs in Lusatia)

 25 Ptaškowa swajźba ends (Sorbs in Lusatia), свято Домовика (Ukrainian Ridnoveri)

February

 2 Громниця (Ukrainian Ridnoveri, and others), Dubrovnik Carnival (Dubrovnik)

 3 Oleg the Prophet's Day (American Russian)

 8 Swieto Welesa (Rybakovian)

 9 Tydzień Welesa begins (Polish Bożywoj)

 11 Велесов день (Slavic Reconstructionist, and others)

 13 Velja Noc (Modern Rodnovery)

 15 Tydzień Welesa ends (Polish Bożywoj)

 21 Стрибог Зимний (Russian Rodnoverie, and others), Wiosenny Strzybóg (Polish Bożywoj)

 24 Лелечини (Ukrainian Ridnoveri), Будник (Russian Rodnoverie)

 26 Maslenica begins (Modern Rodnovery), Nowolecie (Ukrainian Ridnoverie), Rijeka ends (Croatia)

 28 Велесов круг ends (Russian Rodnoverie)

March

 1 Дней Навй begins (Slavic Reconstructionist), Баба Марта (Bulgaria)

 2 Maslenica ends (Modern Rodnovery)

 3 Prince Igor's Day (American Russian)

 4 Дней Навй ends (Slavic Reconstructionist)

 9 Сорочини (Ukrainian Ridnoveri)

 11 Новоліття (Ukrainian Ridnoveri)

 14 Чистий четвер (Ukrainian Ridnoveri)

 20 Strinennia (Modern Rodnovery), свята Ярила (Ukrainian Ridnoveri)

 21 Perun Kwiecien (Rybakovian)

 22 Rękawka (Polish Rodzimowierczy)

 24 Комоедици (Slavic Reconstructionist)

 25 Благовіщення, Волочільне (Ukrainian Ridnoveri)

 31 Радуниця (Ukrainian Ridnoveri)  

Since I calculated dates for the 2018 Slavic Calendar but the publisher decided not to produce it, I'm putting the 2018 dates for the Slavic holidays here on my blog. Caliburn did produce the 2018 Heathen Calendar and it's available on Lulu. For both calendars, some holidays are the same each year and some have to be recalculated for each year. I have done my best to find holidays and how they are calculated, and to do the math. These dates are converted to the Gregorian calendar system.

Image: Fedot Sychkov folk dance in snow Сычков_Пляска public domain