Translation:Today is Saturday.
"是 is nessecary when you have a sentence that is logically A=B, and A and B are both nouns ... it is not necessary when you have a noun followed by an adjective." http://chinesepod.com/community/conversations/post/12767
Here's another explanation from fluentu: https://www.fluentu.com/blog/chinese/2015/07/09/shi-chinese/
Here's a link you might find interesting: http://www.answers.com/Q/Is_Sunday_the_first_day_of_the_week