Complex sentence: Bolghar is a center of local Islamic movement "Little Hajj" that was popular in Soviet period, due real Haqjj wasn't avoid to Tatarstan's Muslims.
Split into simpler sentences: Bolghar is a center of a local Islamic movement, the "Little Hajj" that was popular in Soviet period. This was due to the fact that the real Haqjj wasn't available to Tatarstan's Muslims.