Usage of 'it is' or 'is it' in questions [duplicate]
"what day it is today" is a noun clause in your sentence, the direct object of the verb tell, and thus cannot be in an interrogative form. so your second sentence is the way to go.
"what day it is today" is a noun clause in your sentence, the direct object of the verb tell, and thus cannot be in an interrogative form. so your second sentence is the way to go.