Rule #1 when working retail: Never, ever, EVER trust a manager in charge of scheduling to pay attention to conflicts.
I have two steadfast rules on scheduling, that I always make known up front.
The first is pretty simple, I don't work between the hours of 8 AM and 3 PM. I am NOT a morning person. I generally wake up at about 2 PM. That makes a 3 PM shift doable. 8 AM... sorry, I am asleep.
The second seems to be more complicated. I will work any Holiday, even pulling double shifts, EXCEPT Halloween. I don't care if Zeus, Loki, Yawah, Allah, and the Flying Spaghetti Monster himself all get together and show up at my doorstep, drop to their knees and/or noodles, and BEG me to work on October 31st, it ain't happening.
And yet I can't think of a single time in the past twenty years that I was NOT scheduled to work on that day. It's not a religious thing. It's not a sacrilegious thing. It's that I am not going to come to work when I am drunk off of my fucking ass. (You might guess by that that I was born on Halloween... and you'd be right.)
And, even worse, do NOT expect a manager who has then scheduled you at a time that you have said you won't work, to do anything about it. They will tell you, on the phone and to your face, that they will fix it, but at least 80% of the time when that moment comes that THEY have fucked up and asked you to work and you aren't there, not only will they say "I didn't know you were supposed to be off," they will also swear that you never pointed this out to them after you were scheduled, and that it is then your fault for not showing up.