From the first sitting of the Constituent Assembly in 1946 to the day the Republic began, 26 January 1950. Two years, eleven months, and eighteen days of work.
16 May 1946
Cabinet Mission Plan announced
The British Cabinet Mission proposed the framework for a Constituent Assembly, indirectly elected by the provincial legislative assemblies. Planned strength was 389 members (296 from British India, 93 from the princely states); after Partition the effective strength fell to 299.
9 December 1946
First sitting of the Constituent Assembly
The Assembly met for the first time in the Constitution Hall, New Delhi. Dr. Sachchidananda Sinha was elected temporary chairman; Dr. Rajendra Prasad was elected permanent President on 11 December 1946. The Muslim League boycotted the first sitting.
13 December 1946
Objectives Resolution moved by Nehru
Jawaharlal Nehru moved the Objectives Resolution laying down the guiding philosophy of the constitution. It was adopted on 22 January 1947 and later became the basis of the Preamble.
29 August 1947
Drafting Committee formed
A seven-member Drafting Committee chaired by Dr. B.R. Ambedkar was appointed. Members: Alladi Krishnaswami Ayyar, N. Gopalaswami Ayyangar, K.M. Munshi, Syed Mohammad Saadulla, B.L. Mitter (later N. Madhava Rau), and D.P. Khaitan (later T.T. Krishnamachari).
21 February 1948
Draft Constitution published
The Drafting Committee submitted the Draft Constitution; it was published for public comment. Ambedkar formally introduced the Draft in the Assembly on 4 November 1948.
26 November 1949
Constitution adopted
The Constituent Assembly adopted and enacted the Constitution. This date is now observed as Constitution Day (Samvidhan Divas), declared in 2015. A few provisions came into force immediately.
24 January 1950
Constitution signed
At the final session, 284 members signed the handwritten copies of the Constitution.
26 January 1950
Constitution commenced (Republic Day)
The Constitution came into full force. The date was chosen to honour the Purna Swaraj declaration made by the Indian National Congress on 26 January 1930 at Lahore. India became a sovereign democratic republic.
2 years, 11 months, 18 days
Time taken to frame the Constitution
Measured from the first sitting (9 December 1946) to adoption (26 November 1949). The Assembly held 11 sessions and sat for about 165 days (some sources cite 166 or 167).
Sovereign
India is an independent state, internally supreme and externally free from foreign control.
SocialistAdded 42nd Amendment, 1976
A commitment to social and economic equality and welfare. Denotes democratic, not communist, socialism.
SecularAdded 42nd Amendment, 1976
The state has no official religion and treats all religions equally. The principle already ran through Articles 25 to 28.
Democratic
Authority rests with the people, exercised through elected representatives under universal adult franchise and the rule of law.
Republic
The head of state (the President) is elected, not hereditary. Every public office is open to citizens on merit.
Justice
Social, economic, and political justice for all citizens, drawn from the Objectives Resolution.
Liberty
Freedom of thought, expression, belief, faith, and worship, secured through the Fundamental Rights.
Equality
Equality of status and of opportunity, with no discrimination and no special privilege.
Fraternity
A sense of brotherhood assuring the dignity of the individual and the unity and integrity of the nation. 'Integrity' was added by the 42nd Amendment, 1976.
Sources: constitutionofindia.net, sansad.in, the Press Information Bureau, and Wikipedia, cross-checked. The number of sitting days is cited as 165 in most sources (a few say 166 or 167).