The first popular Ministry of Tripura headed by Shri Sachindra Lal Singh…

Concept: A “popular Ministry” in Indian constitutional history means an elected Council of Ministers that is collectively responsible to an elected legislature — this is a different milestone from a mere merger/accession date, from a nationwide constitutional event, or from a later reorganisation of the same territory's status. The date on which a territory's first popular Ministry “assumed office” is fixed by when its first elected legislature actually installed an elected government — not by any other date connected to that territory or to the country as a whole.

Application: Tripura's path to an elected government ran through several distinct milestones. It merged with the Indian Union on 15th October, 1949 (an accession event, with administration continuing directly, not an elected ministry). It had already been a Union Territory since 1956, governed through a partly-elected Territorial Council — a body without a full Council of Ministers. Under the Government of Union Territories Act, this Territorial Council was replaced by a fully elected Legislative Assembly with effect from 1st July, 1963, and it was on this date that Shri Sachindra Lal Singh was sworn in as the first Chief Minister, forming Tripura's first popular (elected) Ministry. Tripura was later elevated to full statehood on 21st January, 1972 — by which time an elected ministry already existed, so this later date cannot be the “first” one.

• 26th January is when the Constitution of India came into force nationwide — a pan-India constitutional milestone with no specific link to when Tripura's own elected ministry began.

• 15th October, 1949 is Tripura's merger date with the Indian Union — an accession event under direct administration, before any elected ministry existed.

• 21st January, 1972 is when Tripura became a full-fledged state — a later reorganisation of a territory that already had an elected government, so it cannot mark the FIRST popular Ministry.

Cross-check: Ordering the four dates chronologically — 1949 (merger) → 1950 (Constitution enforced) → 1963 (Territorial Council replaced by an elected Legislative Assembly + first popular Ministry) → 1972 (full statehood) — confirms 1963 is the earliest point at which an elected Council of Ministers actually held office in Tripura, which is exactly what “first popular Ministry assumed office” asks for. Hence, the first popular Ministry of Tripura, headed by Shri Sachindra Lal Singh, assumed office on 1st July, 1963.