Wedding preparations are in full swing at the 200-year-old ancestral home, known as Mundarath House.
"We are all busy as Shashi Tharoor is coming. All his relatives are here and we are going to celebrate Onam together in a big way," said Jayasankari Amma, Tharoor's grandmother.
Tharoor's early morning marriage ceremony will be attended by close family and friends and then the couple is expected to leave for his constituency Thiruvananthapuram for the reception on Aug 23.
"This is a very great occasion for us, a get to together also inspite of the marriage occasion where all the people from all over the world, who are relatives are coming here to participate in it. And also it combines with the Onam, which is also a celebration for Malayalee," said M G Menon, Tharoor's maternal uncle.
Sunanda Pushkar, the daughter of a retired Army colonel, hails from Jammu and Kashmir.
A Kashmiri wedding ceremony will take place in Dubai on August 30 followed by another grand reception in Delhi in September.
Tharoor resigned as a minister over allegations of corruption in winning a cricket league franchise in the Indian Premier League.
The controversy surfaced when Modi revealed on his Twitter feed that Tharoor had advised the IPL management against disclosing the name of Dubai-based businessperson Sunanda Pushkar, known to be close to Tharoor.
Following the controversy, Tharoor tendered his resignation on April 18. Modi was later sacked as IPL Commissioner by the Indian cricket board.