Complex sentence: It is not clear when the first Cossacks communities on the Lower Dnipro began to form, however in early September 1990 the People's Movement of Ukraine organized a festival in the region to the 500 Anniversary of the Zaporizhian Cossackdom.
Split into simpler sentences: It is not clear when the first Cossack communities on the Lower Dnipro began to form. However, in early September 1990 the People's Movement of Ukraine organized a festival in the region to recognize the 500th Anniversary of the Zaporizhian Cossackdom.