"Disgraceful that Govt is ignoring the martyrdom of our only Prime Minister who was killed in office in the line of duty. Oct 31 forgotten?", Tharoor said on micro blogging site Twitter.
The Modi government has declared 31 October as "Rashtriya Ekta Diwas" (National Unity Day) to mark the birth anniversary of country's first home minister Sardar Vallabhbhai Patel.
There was no mention of Indira Gandhi in the government announcement though it is also the death anniversary of the former Prime Minister who was shot dead by her bodyguards in 1984.
The Congress had sacked Tharoor as its national spokesperson a fortnight back. His removal had come after the Kerala unit of the Congress took strong objection to his continuous effusive praise of Prime Minister Narendra Modi and demanded that the high command take appropriate action against him.
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Tharoor, party MP from Thiruvanathapuram, had come under severe attack from Kerala Congress leaders after he said he felt "honoured" to accept Prime Minister Modi's invitation to join the Swachh Bharat Mission.
The KPCC's argument was that Tharoor had been repeatedly praising Modi. Days after Modi was sworn in as Prime Minister, Tharoor said his comments so far has been "very conciliatory and inclusive".
Later, he suggested Modi has evolved into Modi 2.0 and argued that the Congress should not be "churlish" to ignore his inclusive outreach.