When?
This festival is celebrated on Jyeshta shukla Amavasya
Mannu means mud; Yethu means bullock. This festival is celebrated in Karnataka, and AP, and bullocks made of mud are worshiped by the farmers
How to celebrate?
People make two bulls out of Mud and Do pooja to the bulls with arshina, kumkuma, flowers and Do neivedhya of Karadakadubu , sweet dishes etc
In a way , this festival of bullock symbolizes the importance of bulls in Farming and It is like thanking the bullock for good harvest
Madhwa 
Hare Srinivasa,
These Mannettina picture are copied from my blog.
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I am sorry. One of the viewer has shared it with me
I have removed pic. Thanks.
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