I echo Melodist's statements, and also:
1. Alexandria should conform its practices to those of Rome.
I think he might be making an argument for his way of doing things, yes. I do have to wonder what prompted St. Leo to write this letter? It doesn't say, and he really makes no diplomatic or introductory remarks, only a salutation. Go figure.
2. Priests should be ordained on Saturday night.
I think he's actually making an argument for ordaining clergy on the Lord's Day, and is simply including Saturday evening in the keeping of the Lord's Day, as that is the ancient custom even up to the present day. St. Leo writes, "...that those who are to be consecrated should never receive the blessing except on the day of the Lord’s resurrection, which is commonly held to begin on the evening of Saturday,"
I do have to wonderful if, in Rome, their clergy were ordained during the Liturgy at the time St. Leo's was pope? I don't think they do currently (someone please correct me if I'm wrong), but I have no idea about back then. If they did not, then St. Leo could be making an argument for ordaining clergy on Saturday evening after Vespers, which would be liturgically Sunday.
3. On feast days, churches should have more than one liturgy.
Now, I do know that today's Roman Catholic Church doesn't hold to the "one liturgy per priest per altar per day." rule. Their rubrics sometimes actually call for multiple Masses to be served on the same day, such as certain Solemnities. For example, I believe they hold something like four Masses on Christmas day (midnight, morning, afternoon and evening...or something like that). I think on any given day, at least two Masses may be served. Again, I do not know if this was the case during St. Leo's time, but it's interesting to think about.
1. What was the context in which this letter was written?
2. What was St. Dioscorus' response?
Yeah, no idea on both counts here.
I'd like to know myself! I'll see if I can find more information on this letter, but it'll have to wait until tomorrow...it's past midnight here and I'm crashing soon!
Also,--and this could be due to my bias as an OO--it seems to me that Pope Leo thinks he is in a position to tell St. Dioscorus what to do.
I don't know. I think he's advocating his way of doing things and that he thinks its proper. He's not ordering him to follow these practices, though.
He seems to refer to himself as "the teacher," and to St. Dioscorus as "the taught":
We do not therefore allow it that we should differ in anything, since we confess ourselves to be of one body and faith, nor that the institutions of the teacher should seem different to those of the taught.
And in his very last sentence, it seems to me that Pope Leo is claiming a sort of exclusive Apostolic authority.
Again, someone let me know if I am way off on this. And again, I would like to know if St. Dioscorus ever responded to this particular insinuation.
I think he's making an argument for unity here, saying that St. Mark (founding bishop of Alexandria) should be in agreement with the Tradition has it was passed down from St. Peter, since he was his disciple. I think he's making an argument for Alexandria to adopt the practices of Rome. Why? No idea. However, I don't think he's referring to any exclusive authority of Rome, I think he's just making an appeal for the shared Petrine origins of both Rome and Alexandria, as I said in the Free-For-All thread you've referenced.
I don't believe this letter can be used to support the idea of Papal Supremacy as dogma, or that St. Leo is even trying to exercise some form of supremacy.