Hi, there. Welcome to St. Louis. Just remember not to ever say St. Looie or Missoura, and you'll be fine.
Here's a link to the Archdiocesean site.
I found you some Masses at 6am on the southside because we're sort of assuming you're downtown. If you're going to be in some other part of town, let us know. I'll be happy to try again. If you'd like a Mass in some language other than English, the website has a link under parishes for Mass times, and one of the options is language. We have Spanish, Viet Namese, Hungarian, Croatian, Lebanese (that's St. Raymond's which is Eastern Rite), Korean, and of course, several churches have Latin Masses. You name, we got it.
St. Francis DeSales which is just south of downtown on 12th Street (but it's Gravois by then) has Latin Masses. Here's their website: institute-christ-king.org/stlouis/
Here's the New Cathedral, as we call it, which isn't far from downtown, but the earliest Mass there is at 7am. This would be a great place for your Sunday Mass. You feel like you're really "in church" here. cathedralstl.org/site/index.php?option=com_content&task=view&id=18&Itemid=2
Here's the Old Cathedral, which is one of the historical gems of the city. It's smack on the waterfront, under the Arch and across the street from the courthouse where the Dred Scott Decision was made. They have Mass at 12:10, and although I haven't worked downtown in many years, it used to be pretty fast, usually no singing, so people can get there and back to work on their lunch hours. I used to always get ashes there. It's also very beautiful in a sort of elegant rustic way, if that makes any sense. psichurch.com/churches/140stlouis/
There are a lot of other parishes near downtown that are architecturally interesting. St. Peter and Paul comes to mind, on 7th. You'd just head down Broadway and it's a few block after the stadium and before the brewery, two other sites St. Louisans cherish! They used to have a nice tour at the brewery which is quite beautiful inside. Since the new ownership, I don't know if they still do that.
The old Holy Family Church in Cahokia IL is a great place to visit. It was built in 1699 by Canadian missionaries and is the oldest continuous parish in the US. They have a Latin Mass at 9am on Sunday (my buddy Christine plays the organ sometimes). Here's their website: holyfamily1699.org/. Not far from there you'll find the Cahokia Mounds, where the Cahokian Indian tribe lived. There's a nice museum, and you can climb the mounds too, but it might be too cold in March. I wouldn't want to take a cab to Cahokia, so if you don't have a car, this and Our Lady of the Snows should be saved for another day. snows.org/ It might be too cold for the outside Mass, I don't even know if they have it that early in the year, but the altar is built into the side of a hill. Around the back there's a place with candle, and there are big gorgeous Angelus Bells in a pond, and it's all just lovely. The church is very unusual, modern, but none-the-less beautiful. I've only been to Mass there when I was on a cantor's retreat, but the way the light hits the altar on a nice day is just stunning. But if you don't like modern church architecture, this place might not hold the same charm for you as it did for me. We have churches of every stripe around here, from 1699 log churches to brand spanking new St. Gianna's in St. Charles.
Have fun. There are some great places to eat on Laclede's Landing. The hotel is sure to have some brochures for you. If you're in Webster Groves, I'll probably be singing at the 9am Mass at Holy Redeemer.