The event that triggered the February Revolution in Russia was the "Bloody Sunday Massacre."

The Bloody Sunday Massacre was a significant event that occurred on January 22, 1905, in St. Petersburg, Russia. Thousands of workers, led by Father Georgy Gapon, a priest and labor leader, marched to the Winter Palace to present a petition to Tsar Nicholas II. The petition demanded better working conditions, higher wages, and an end to the Russo-Japanese War. However, instead of addressing their concerns, the peaceful demonstration turned into a tragedy.

The event unfolded as follows:

1. The Protest: On that fateful day, around 150,000 workers, their families, and sympathizers gathered in a peaceful protest. They carried banners and icons, singing religious hymns and calling for the Tsar's attention.

2. The Massacre: As the crowd approached the Winter Palace, they were met by a line of soldiers who were ordered to prevent them from entering. The protesters continued to advance, believing they would be heard. Suddenly, without any warning, the soldiers opened fire on the crowd. Estimates suggest that around 200 people were killed, and hundreds more were injured.

3. Public Outrage: News of the massacre quickly spread throughout Russia, causing widespread shock and anger. The event became a symbol of the government's cruelty and indifference towards the people's suffering.

4. Impact: The Bloody Sunday Massacre had a profound impact on the Russian population. It intensified widespread discontent and deepened the divide between the ruling elite and the working class. It also fueled the growing revolutionary sentiment and led to an increase in strikes, protests, and uprisings across the country.

5. February Revolution: The Bloody Sunday Massacre acted as a catalyst for the February Revolution of 1917. It galvanized the masses and further eroded the legitimacy of the Tsarist regime. The event sparked a wave of protests, strikes, and mutinies that eventually led to the abdication of Tsar Nicholas II and the end of the Romanov dynasty.

In conclusion, the Bloody Sunday Massacre played a pivotal role in triggering the February Revolution in Russia. The violent suppression of peaceful protesters by the Tsarist regime not only resulted in numerous casualties but also fueled public outrage and contributed to the revolutionary fervor that ultimately led to the downfall of the monarchy.