"Thackeray will pen the letter in Marathi and MNS workers will go to households in the state on February 27 to deliver the letter," a leader of the four-year-old party said.
He added that the letter will carry instruction on using Marathi everywhere.
"In the letter, the MNS chief will urge the people to use Marathi everywhere, including offices, trains and marketplace," he said.
Marathi Language Day is annually observed on Feb 27, in memory of noted poet VV Shirwadkar alias Kusumagraj.
The letters will be delivered by MNS to every household, including non-Maharastrian.
This move by the MNS can be seen as subtle threat to non-Marathi speaking people to learn the language or be prepared to bear the Sena' s wrath.
In its massive campaign to propagate Marathi, MNS had blackened all the English signboards across the city, demanding signboards in Marathi, two years back.