Lord Vishnu’s 10th avatar already gets celebrated as “Kalki Jayanti”. This day is considered auspicious as everybody celebrates the coming avatar of Lord Vishnu.
Lord Kalki will be born on the thirteenth day during the fortnight of the waxing moon. A waxing moon is a progressing moon when the moon goes from Amavasya to Purnima night.
It has also been told in ShivPurana that Lord Kalki will worship Lord Shiva. And because of this, he gets all the godly powers to defeat evil.
Lord Rama mentioned his 10th avatar as Lord Kalki in Kalyug. It is believed that since that time, Vishanvi has been praying and worshipping to unite with Lord Vishnu.