What is a Kumbhabhishekam and a Maha Kumbhabhishekam, respectively?
I am quoting from Wikipedia, because I couldn't say it any better:
Kumbhabhishekham, also known as Samprokshanam, is a Hindu temple ritual that is believed to homogenize, synergize and unite the mystic powers of the temple's deity. Kumbha means "the Head" and denotes the Shikhara or Crown of the Temple and Abhisekham or Prokshanam is ritual bathing. Kumbhabhishekham is widely celebrated as a festival in South India and especially in the State of Tamil Nadu.
On the appointed day and at an auspicious time, the Kumbha is bathed with the charged, holy waters sanctified by the preceding yagna. By a mystic process, these pranic powers trickle down a silver wire and enter the deity installed inside the sanctum sanctorum of the temple. The deity, which was until then only a granite sculptured stone image, is believed to transform into a vibrant and vivid living representation of the deva with innate beatitude, grace and grandeur, conferring divine blessings on all devotees.
A Kumbhabhishekam can (or should) be performed annually for a temple deity. A Maha Kumbhabhishekam is performed in a grand fashion every twelve years to honour and renew the spiritual powers of the deity in this specific temple.
Below are pictures of the Maha Kumbhabhishekam performed in a simple way for the "Shankha Vinayaka" Temple in the Aum Pranava Ashram on the 28th January 2018.