These holidays are drawn from various Slavic traditions and nations. They are converted to the Gregorian calendar. 

July

 1 Rusalle (Minsk)

 3 Day of Remembrance for King Sviatoslav the Great (American Russian)

 4 Kupala Night (Belorus)

 7 Ivandan (Modern Rodnovery)

 14 Стрибог (Ukrainian Ridnoveri)

 20 Днём Перуна (Slavic Reconstructionist, and others)

 22 Перуниця (Ukrainian Ridnoveri)

 27 Чур, Пращур (Ukrainian Ridnoveri)

 

August

 1 Медово-Маковий Спас (Ukrainian Ridnoveri)

 5 Dan Domovinske Zahvalnosti (Croatia)

 6 Яблочный Спас (Russia, and others)

 7 Day of Veles and Mokosh (American Russian)

 15 Спрошинкы (Slavic Reconstructionist), Veliku Gospu (Modern Rodnovery)

 16 Свято Лісовика, Хлібний Спас (Ukrainian Ridnoveri)

 18 Flor i Laur (Belorus)

 19 Посвистач Стрибог (Ukrainian Ridnoveri)

 26 Zadar Night (Zadar harbor, Croatia)

 28 Medjudnevnica begins (Modern Rodnovery)

 

September

 1 Свято Вогню (Ukrainian Ridnoveri)

 3 Day of Remembrance for Novgorod (American Russian)

 4 осінні Перун і Перуниця (Ukrainian Ridnoveri)

 6 Tsuda (Belorus)

 8 Day of Lada and Leyla (American Russian), Рожаницям begins (Ukrainian Ridnoveri)

 9 Рожаницям ends (Ukrainian Ridnoveri)

 14 свято Вирію (Ukrainian Ridnoveri)

 22 Днём Сварога (Slavic Reconstructionist), Dožínky (Modern Rodnovery), Święto Plonów (Polish Rodzimowierstwo)

 25 Правилася (Ukrainian Ridnoveri)

 

Special thanks to Scott Simpson for providing documents on which I based some of my research for the Slavic calendar. 

Image:  Rusalki by Witold Pruszkowski