Hanuman Jayanti

Anjana, cursed to be born as a vanara, performed austerities to Shiva. Vayu carried a share of the sacred payasam from Dasharatha's yagna to her, and Hanuman was born, Vayu's son and, in Shaiva accounts, an incarnation of Shiva.

Hanuman Chalisa recited, often forty or a hundred and eight times. Sindoor and oil applied to the idol. Red flowers, and a bundi or laddoo offering. Bodybuilders and wrestlers hold akhara observances. Saturday and Tuesday are his days generally.

Bundi laddoo, jaggery, chana, and panjiri. Many devotees eat once and avoid salt.

As above. The date itself is the variation, which is unusual. That some regions observe it in April and others in December is not an error in anyone's calendar.