Translation:The day before yesterday was Sunday.
The emphasis is perhaps different. Arguably your sentence is more likely to answer the question "when was Sunday" instead of the question "what was the day before yesterday".
Assuming that "星期天是前天" is a valid Chinese sentence, I wouldn't really advise using your suggestion to translate Duo's sentence.
Chinese native speakers?
To be clear, though, "很" doesn't come into play with this particular sentence.
"很" (literally "very", but sometimes described as a sort of copular filler similar to "to be") is used with an adjectival subject complement, whereas "是" (literally "to be", in whatever form of the latter is appropriate) is used with a nominal subject complement.
Yesterday is 昨天 and the day before yesterday is 前天; 2 days before yesterday is 大前天. We don't say them like the day before xxx literally which sounds awkward in Chinese.
For the day before a specific festive or memorial day we say xxx 前夕, e.g. 圣诞前夕 Christmas Eve.
For the day before an ordinary day, we say ~ 前一天, e.g. 我们出发前一天 The day before we set off (a trip). (This is only an instance and other variations exist.)
What day (of the week) was it the day before yesterday?
So, if you don't know what today is then you won't know what day of the week it was two days ago, but if you are in a conversation where maybe others are discussing the strange hours a shop was open that day you can ask the above question.