how to say "today is the last day of [something]" in korean?
would it be
오늘 마지막 날입니다?

Question

how to say "today is the last day of [something]" in korean?
would it be 
오늘 마지막 날입니다?

Answer

Yes, you are correct!

오늘(은) 수업(의) 마지막 날입니다
Today is the last day of class/course
오늘(은) 여행(의) 마지막 날입니다
Today is the last day of travel

You can say 오늘이, or simply omit them and say 오늘.