The reason they don't give you a definite day and/or time to report to work is because they will only call you when they need someone to come in because, for example, another employee called off, etc.
(They aren't going to tell you this though.)
As someone mentioned, it doesn't hurt to just show up at whatever the start time is, but you may be sent home. This sucks if you don't live close by.
Regarding training -- me and any other new hires had to arrive 15 minutes early for 15 days for training. (I mean days that we were called in to work, not necessarily 15 days in a row. When you're new, you may only work two or three days per week for the first month.)
We had to wear a mesh, blue vest all day, and we covered one safety topic/rule/procedure per day and then signed a sheet to prove we completed that day's topic.