In Bereshit we read "the evening and the morning were the second day ... the third day ... the fourth day ..." etc. Now in the Berlitz Hebrew phrase book "yom sheni", translated "second day" in Bereshit, is simply rendered "Monday", and so for the other days (apart from day one). My question is - would it be legitimate to use "Monday", "Tuesday" etc when translating Bereshit into English?

I am not an expert of the English language. The words Yom Sheni (as in firs Perek of Bereshit) mean literally – the second day; we call the days of the week by there relation to Shabbat i.e. first second etc. The English world, calls these days by random names (Sunday Monday) which correspond to the "first and second etc." of our week. (Ramban Shmot 20; 8)