This commemorates the return of Ramachandra to Ayodhya after He had conquered over Ravana and rescued Sita-devi. The Ramayana states:

Lord Ramachandra entered Ayodhya in the midst of a great festival. He was greeted on the road by the princely order, who showered his body with beautiful, fragrant flowers, while great personalities like Lord Brahma and other demigods glorified the activities of the Lord in great jubilation.

Lord Rama was dressed up in royal style and was decorated with garlands and ornaments. Lord Rama mounted a lavishly decorated chariot and Bharata took up the reins, while Satrughna held the royal white umbrella. On either side stood Vibhishana and Lakshmana, waving a fan and a chamara. As Rama proceeded towards Ayodhya, a huge procession followed Him, while all the monkeys, appearing in human form, rode elephants. Men and women, children and the elderly gazed upon Rama as though they were getting back their lost lives. While waving their clothes and jumping with excitement, the people shouted, “ Our beloved prince has returned! All glories to Lord Rama, the maintainer of His devotees.”

At Mayapur, murtis of Lord Rama, Lakshmana, and Sita-devi are taken out on a colorful procession in the midst of kirtana. Oil lamps are placed all through the campus to welcome Their Lordships following in the footsteps of the residents of Ayodhya. Fireworks abound, and rangolis are put up to receive the Lord.

During this month of Kartik the devotees celebrate the pastimes of Lord Krishna as a young child. Damodara-lila is a sweet pastime of the Lord showing us how He could be bound by the love and affection of His devotees. Every evening throughout the month, the temple is bathed in the warm glow of little lamps. The devotees offer ghee lamps to the accompaniment of Damodarashtaka.

Source: Mayapur.com