Original: The custom is said to have started during the Nara Period (710 - 794) when it was ume blossoms that people admired in the beginning. But by the Heian Period (794 - 1185), sakura came to attract more attention and hanami was synonymous with sakura.
Simplified: Though it was ume blossoms that people admired in the beginning, by the Heian Period (794 â "1185), sakura began to attract more attention.