When and how did India gain independence from the British Empire?

The Indian Independence movement was a long and difficult journey for the Indian people. The push for Indian autonomy began in 1858 when the Indian National Congress was founded as a forum to express nationalist feelings. In 1920, Mahatma Gandhi became the figurehead of nationalism in India and would ignite the fire that would develop into a national movement for independence.
Mahatma Gandhi preached non-violent civil disobedience. His motive was to protest the British by using methods that did not promote violence. He taught that by acting violently you were not making progress, but showing the British that you are acting in the way that they thought you were; a savage or an animal. By using non-violent methods, Indians were able to cripple the British using boycotts against British goods like tea and salt to hurt them economically. From 1920 to 1922 Gandhi launched his anti-British civil disobedience campaign. During these years he orchestrated the people of India to make their own cloth, trade local goods and boycott any British import.
One of Gandhi's greatest successes was the Dandi Salt March. On March 12, 1930, Gandhi led a civil protest against the Salt Mine that was owned by the British. Gandhi along with his followers marched 240 miles along the western coast of India. His goal was to march to the coast to collect salt from the sea. This was a major act of civil disobedience because the British imposed a Salt Tax in 1882, which prohibited Indians from collecting and or selling salt. This tax hurt the local Indian economy and within their own culture because salt is a staple in Indian cuisine.
After the success of the boycotts and the Salt March, the Indian National Congress launched the Quit India movement in 1942. This was at the height of WWII where the British were being weakened by Hitler and the Nazi forces in Germany. With Britain being vulnerable this was an opportunity that Gandhi saw to push further for independence. There were various protests and the British could not control it. In 1944, Gandhi was arrested and spent his time participating in a 21 day fast. This made international headlines. At the end of WWII, Britain's position in the world had changed forcing them to relinquish control over India and proceeded with independence.
On August 5, 1947, India gained independence from Great Britain and India and Pakistan would begin their conflict over territory, political and religious differences.

Resources:
https://www.bbc.com/news/world-south-asia-12641776

British rule in India lasted for 190 years from 1757-1947.
Revolt against British rule started with “India’s First War of Independence” in 1857 when soldiers in Meerut refused to accept their cartridges and tear them with their teeth. However this movement was suppressed by rulers within a year. So the history of “Indian Independence Movement” started with this rebellion.
Later the Non- Cooperation Movement was launched on August 1, 1920 led by Mahatma Gandhi after Jallianwala Bagh massacre killing thousands of protesters. Mahatma Gandhi advocated nonviolence throughout this movement and also in his life. He called for nationwide protest against Rowlatt Act by leaving Government services and boycotting foreign products. This movement ended with violent ‘Chauri Chaura’ incident when Mahatma Gandhi felt that the very essence of this movement i.e. Non- violent protest is lost. It was withdrawn in year 1922.
The Second World War started in 1939 and British Governor General announced India’s participation in war without consulting Indian nationalists. In 1942, Mahatma Gandhi started another movement called “Quit India Movement” with the slogan “Do or Die”. After the war, an Interim Government under the chairmanship of J. Nehru was formed in 1946. However, The Muslim League refused to be a part of it and demanded a new state for Muslims. Lord Mountbatten, last Viceroy of India proposed the scheme of division of India into India and Pakistan.
So finally India got its independence in the midnight of August 14, 1947. India celebrates its Independence Day on August 15 every year.

There had been Indian independence movements going back to the mid-19th Century. Most of these were violent revolts that were brutally represented by the British military. The modern Indian independence movement began in the 1920s under the leadership of Mahatma Gandhi. Gandhi believed that independence would best come through a non-violent civil rights struggle against Great Britain. This consisted of many acts of civil disobedience, such as refusing to pay British taxes, and peaceful demonstrations to make the independence cause heard.
After World War II, Great Britain found itself in a weakened state. The British had promised to liberate India after the war if Indians aided in the war effort. Calls for Britain to keep their end of the bargain were renewed in the late 1940s. Prime minister Clement Attlee decided to uphold the bargain. India was partitioned into the mostly Muslim Pakistan on August 14, 1947, and the mostly Hindu India received independence from direct British rule the next day. India nominally remained under the sovereignty of the English Crown for the next three years, until the Constitution of 1950 was ratified giving the nation full autonomy.
https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/i/Indian_independence_movement.htm