If I'm a 42A, do I work Monday through Friday or will I be working on weekends?

You are a Soldier as a 42A.
You will work whenever the Chain of Command says you work.
Might be bankers hours. Might be many hours.
As for PT, different units do different things, but you can reasonably expect to report for PT in the morning.

Johnathon Greer you keep looking at this like a clock punching job. Shake your mental Etch-a-sketch.

42As largely work in S1 sections. They handle personnel actions like pay inquiry, awards, strength reporting, unit status reports, Manning/slotting, personnel actions for individuals (processing DA4187s). You would work in a Battalion, Brigade, or Division staff at the direction of your NCOIC or staff primary. Sometimes you work a “normal” duty day: 0630 formation, PT,0800 personal hygiene, 0900 work call, work, dismissed for the day 1700. Sometimes you are training, doing manifests for deployments, SRP, etc you work a shift or continuously until the mission is done. You’ll pull 24 hrs CQ or staff/runner/driver duty when you come up on the duty roster. You’ll get put on different work details as assigned. You’ll pull weekly maintenance on your unit equipment.

HR Companies are mostly 42As. They do personnel theater gateway and mail. Similar to above except what they do at work call.

All of the fore mentioned and, you may be assigned as a vehicle driver and work one or more days out of a motor pool, not to mention you have to certify in basic Soldier skills periodically as established by the command. It sounds as if you want to be a clerical person at a local business, the Army is far from that. If you are motivated by teamwork, fitness, and duty-honor-country you will do well. If you are motivated by working an 8 hour shift and individuality you should consider another profession.