In chapter 604 of the manga, it is shown that Kakashi kills Rin. This fulfills the requirement to activate the Mangekyou Sharingan. So that should have been the moment when Kakashi was able to use the Mangekyou Sharingan.
Also, the Yellow Flash was still alive when this happens, so this means that Naruto was not yet born. This clearly points to the fact that from the very beginning of the Naruto series, Kakashi had his Mangekyou Sharingan activated.
When Kakashi first fought Itachi (when Itachi and Kisame appear at Konoha) and faced the Tsukuyomi he was devastated in a moment, even though he had his Mangekyou Sharingan active (though not used). But the next time they face (during the rescue mission of Gara), he was able to withstand it.
Now my first question is, was Kakashi unaware of his Mangekyou Sharingan until he faced Itachi, or was he not proficient in using it. Also why in the Naruto series (I mean the series before the start of Shippuden) didn't Kakashi use his Mangekyou Sharingan?
Now for my second question, in the same chapter, a few pages back, just when Kakashi strikes the blow at Rin, Obito's vision starts to fade as he can sense Rin dying. Does this mean Obito's Mangekyou Sharingan was awakened at the same time? We know that Kakashi and Obito shares the same "other dimension" because of the same Sharingan that the both use (not the same eye, but the eye of the same Uchiha). So does the same apply for their activation of Mangekyou Sharingan?