I know a turkish sentence is in the order of subject, object and verb. If you are using a day of the week or month in a sentence, are they used first as a subject ???

 Yes you can use them first like

Bugün işe gidiyorum...today im going to work

But Turkish language is flexible

So also you can say,

İşe bugün gidiyorum

İt depends on what do you want to make the stress on

But this doesn´t make ´bugün´ the subject of the sentence - the subject is still ´I´.

If a day or a month is the subject of the sentence, why would they be excluded from the rule? If they are the subject, they´ll come before the verb.

 I was referring to the sentence ´Bugün işe gidiyorum´ - in that sentence ´bugün´ is not the subject.

Yes,i didnt say it makes it the subject,i said,it can come at first .

Btw:why my post is shaded and how !

Yes,i didnt say it makes it the subject,i said,it can come at first .

Btw:why my post is shaded and how !    

I know - I was answering mltm´s post

No idea why your post is shaded - I was wondering the same thing!

 I was not referring to you. You are right. I said it to the asker, but I think I got the question wrong. I think he asks if it comes first, does it mean that it is a subject. No, as CANLI said, they can come first but as adverb.