My mom, who is in the middle (moderate) stage of Alzheimer's, gets increasingly frustrated about not knowing her daily schedule. Any thoughts?

You can move it to a more prominent place, but that might increase the questions. She may not be able to read it anymore, and all those markings on the board are incomprehensible and she knows it has to do with where she's supposed to be but how can she understand if you're writing what appears to be gibberish? And the more she gets agitated about it, the more she's confused. This is how they start thinking. Or not thinking.

At some point you'll have to abandon the whiteboard and tell mom what she needs to know just before she needs to know it. Such as, "We'll be leaving for the dentist's office in fifteen minutes. Let's go put on your nice shoes." Minimize conversation about how she went to see Aunt Gertrude yesterday and how the grandkids will be over next week. (Who is Aunt Gertrude? What grandkids? What is a week?) They increasingly live in the present. Even chance comments by dad about how there was an earthquake in California yesterday can confuse. (What is an earthquake? Is California where I live? Yesterday?) For that reason, keep small talk to a minimum. Yes, that's a loss of companionship, but others have to accommodate to their brain, not her brain to yours.

Medicine to calm her down might help a lot.

Often times folks with dementia lose their ability to read and to comprehend the written word and even lose their ability to understand the spoken word, so I'm not so sure that moving the whiteboard calendar to "a more prominent place" will do any good either.
If she is constantly agitated I would talk to her doctor about putting her on medication for that as there are several available to help with agitation. And just tell her day of if she has anything going on that day.
I mean how many things can a person with dementia have going on anyway that you'd need a calendar? Her life should now be kept as simple and conflict free as possible, especially if all this "going" is causing her such anxiety.

During mid-stage dementia I tried putting info on my mother’s calendar plus bought a digital calendar clock. I also displayed a sign on the days when a woman was coming to clean her house, do her laundry and prepare her lunch. No matter how much I explained, nor how simple and obvious I made it, she lost the ability to connect the dots. Which made her angry, as she was certain we weren’t telling her anything. There may no longer be a way to keep her informed.

She may be past that stage where a white board helps her. With dementia eventually they don’t understand the written word.

I used post it notes on the cabinets when my spouse would forget where things were. However eventually the notes became meaningless to him. I went with pictures ( a simple drawing ) of what was in cabinets. That helped for a bit, then eventually it didn’t. If she can’t remember to check white board or doesn’t understand what’s on white board, quit using it. Don’t expect her to learn it.

Or you could try just putting one thing on the board at a time…like whatever is next in her day. But that’s harder for you to keep up with. And moving the board to a prominent place also.

Again, she may be getting past that. Sadly, agitation is part of dementia. You’d be agitated too if you wanted to know something, but just couldn’t understand what you were seeing or being told.

Just be patient and loving with her the best you can. Dementia doesn't allow for many answers to the things we run across as caregivers.

Can she still read easily? or know that she is supposed to read the sign?
I'd place it anywhere she would see it easily. Unsightly or not.

She needs structure. Greet her when she wakes and say what is on the agenda for the day.. At breakfast, say what is on the agenda for the day. Whenever she asks, say what's on the agenda. Don't get frustrated, if possible, just because you are repeating yourself.
Her brain is dying and apparently the short term memory section is gone.

She is over the whiteboard stage, we had one for our step-mother before she was moved to MC, all it did was make it worse as she could not follow it.

She no longer understands the written word. You are trying to force her to understand, she has lost the ability to link things together, all this whiteboard stuff is frustrating her, it is like making her take a test, one that she will fail.

Just tell when needed and forget about trying to keep her informed of a schedule, what does she need one for anyway?

I am sure her daily schedule is basically the same day in and day out.

My mom enjoyed seeing details on her calendar with the name of the person visiting.

Have you considered trying something suitable for children with pictures for activities? Often you can find stickers to put on a calendar instead of words.

Make a colorful daily schedule with pictures.

If she is still able to read and comprehend just not retain maybe rather than a calendar that has multiple days and info switch to a white board on which you can put just the info for the day each morning or the night before. Maybe the day of the week written out Lund date at the top and then and appointments or visits she has scheduled with as few words as possible and each with a line of its own well separated. The board could be placed in a place of prominence for her habits so in her room if that’s where she spends most of her time, in the kitchen if it’s the thoroughfare, the hall way or her area in the living room wherever it is that makes the most sense in her day and her access. Where will she look for it first not where you and your father try to train her to look and keep in mind that the spot that makes sense to her brain today may not be the same spot that makes sense next week. On the other hand you might be lucky! This also probably won’t work for ever but it might help her just knowing it’s there and she will satisfy herself that she understands it. If the time comes that she insists something wasn’t on the days info that is ie; granddaughter visit…”I didn’t know she was coming today” “It’s on the schedule” “no it isn’t” don’t fight it just go with it “your right I wasn’t clear I’m sorry” trust me this will be so much easier for both of you.

A person with moderate dementia won't be able to pick up a new routine. (Yup, I did a whiteboard too, on Mom's door. Total fail.) If I need to go somewhere with her I just give her 30 minutes to get ready & we're off. Anything else creates repetitive questions.
Have you tried not discussing a schedule at all, or if she asks, telling her she has nothing scheduled for the day?
Her agitation probably isn't about the schedule so much as it's frustration with how out of whack things seem to her. She could be trying to balance out the effects of the disease by wanting the structure a schedule provides, not realising that she can't recall anything around it.

My mom was always a monthly calendar kind of gal, so has kept on with a paper pocket calendar that has birthdays and holidays marked on it. (no appointments or events). She crosses off each day. I think having something she can carry gives her a physical cue. Visual cues stop working after a while.

If your mom persists, can you divert her with her some repetitive tasks, like folding laundry? If you think that she's focusng a bit too much on the schedule, like an obsession, perhaps talk to her physician for some low dose anti-anxiety medication?

I'd suggest taking pictures of her doing routine activities, as well as some simple ones like being at the store or at her doctor's office or having a cup of tea or coffee with a favorite relative. Laminate them (if desired) and glue a loop on top. Put a word label on if that's meaningful. Tack the appropriate ones to a cork board in the order they'll happen each day. The loop lets you turn them over easily as the "event" is complete. That way you can look at the cork board together to see what is next.

Pictures of things like teethbrushing, shower prep, breakfast, preparing for bed can be used every day, of course. This can help with reminders about daily routines as well. After a while, there's a shift so it's not you telling her what to do. In a way, the "schedule" becomes the boss.