'Tomorrow will be Monday' is the correct translation.Unfortunately, it has crept into modern usage in English to ignore tenses in sentences like these and to use the present tense, hence 'Tomorrow is Monday' may often be heard, even though it is incorrect in a grammatical sense.
I just completed this exercise with select the words in English. CONFUSED !!!!!!! When I look at the dictionary clues for биде I get "will be/will/there will be" The word "IS" as used in the "correct" answer has the definition "present tense third person singular of BE". As "tomorrow" is inherently future tense the use of "is" as a substitute for "will be" is inappropriate. Although this is a common mistake in "lazy speak" or colloquial language, it is however incorrect.