It seemed that the Communists, like all opponents to the Czar, were galvanized by the protests of Bloody Sunday. The dissenting voices to the Czar had become quite loud and persistent, given the fact that that Czar's troops had fired on working women and children. The Communists were able to seize on this and use it as representative of how the Czar really felt about his people and how there had to be an overall change in authority in order for the plight of working people to be recognized. Bloody Sunday also displayed how completely out of touch the Czar was, as the protesters were asking for some level of acknowledgement from their impoverished status. The Communists were able to play the event as an example of the wealthy, the ones who owned the means of production, never acknowledging the voices of those who toil and struggle for so very little.
When the Bloody Sunday protests happened in 1905, the communists were not the main force behind them. But the communists used the protests to help them gain some amount of power. Specifically, they started to organize "soviets" in St. Petersburg in the later parts of 1905.
Before long, the soviets were outlawed and suppressed, so it's not as if there was this unbroken move toward power for the communists. They really weren't heard from again until 1917, but the 1905 soviets gave them a model to use for 1917 when they actually had the chance to come to power.
Bloody Sunday refers to the event on January 22, 1005, involving firing by troops of Russian Czar on a crowd of about 20,000 unarmed protesters who had assembled for a peaceful march on the winter Palace in Petersburg. The proteste4rs had planned to present their charter of demands to the Czar, Nicholas II. Hundreds pf people were killed or wounded in this firing. This incident cause a rise in resentment of people against the Czar and strengthened, and the Russian Revolutionary movement gained much strength.