Do Ismailis observe the Day of Arafat?

for Ismaili Muslims generally, the exoteric Hajj is not of primary importance, nor are the rituals related to it like slaughtering sheep, etc. Therefore, at the Jamati level, fasting on the Day of Arafat is not observed, nor is the Hajj to Makkah undertaken as an obligatory Tariqah activity. Sunni and Twelver Muslims fast on this day to get forgiveness of sins. Ismailis have their own daily and monthly rituals for forgiveness of sins which are more than adequate for the present time. 

The esoteric or batini Hajj is paramount for Ismailis -- read more about this at the below links:

short version: http://ask.ismailignosis.com/article/46-hajj

full article: https://ismailignosis.com/.../esoteric-hajj-from-the.../