Sheetala Saptami (March 8) is observed to worship the goddess Sheetala Mata, mostly in the rural areas. People worship Sheetala Mata because they believe that she will take care of their family from chicken pox and smallpox.

Sheetala Saptami Puja Vidhi
People who worship Sheetala Mata, before going for prayers, take a bath with the cold water early in the morning. If you live in a metro city then take a bath at your home and then start your puja and if you live in the rural area than the most appropriate way of doing puja is to go and take a bath in the river and install the image or murti of Sheetala Mata at the bank placed on a red cloth and read Sheetala Astakam. Sheetala Astakam is read on this day for the welfare of the people. The puja dwells sixteen types of offerings known as Shodasopachar.

Make sure to not to light any diya or torch because Sheetala Mata represents coolness, therefore no fire should be used during the puja and in the homes or kitchens as well. Due to this rule, people cook their food a day before on Sashti, also known as Randhan Chhath. 
In some places, raw flour mixed with molasses is used to make lamps (diya) and lit before Sheetala Devi with ghee and cotton wicks.

Sheetala Saptami Fasting rules
Normally there is no rule of fasting on this day. People eat food which they have offered to the goddess as a Prasad but some people also practice fast on their own will. While practising fast many people sacrifice salt from their edibles or also avoid wearing sleepers as a part of our rituals.

Also, see: 

Sheetala Saptami 2018: Rituals followed by women's on this day