At one point in the video, Alia asked "why do you think we are going to Kumbh Mela?" and Ranbir joked "to leave you there".
On a serious note, the actress said: "It is a special day and it is a supremely special plan from the team of 'Brahmastra'. We are excited about it."
Teasing Ranbir, Alia said: "Give a nice smile with a dimple."
Filmmaker Karan Johar also tweeted: "Tonight, the Kumbh Mela is going to be a lot more magical."
The team will "unveil the logo of the film in a spectacular way", a source in the know told IANS.
"Brahmastra" is a fantasy adventure trilogy. The film by Fox Star Studios also stars Amitabh Bachchan and Nagarjuna.
The unveiling will be held later in the evening.