Diamond day frequency and online order mistakes?

When is the next diamond day? Since I missed out on the GWP I figure I could probably wait for the next diamond day. Are they always the first Monday of the new moth? Or do they always do things on Mondays for diamond members?

Also, does it seem like more and more online orders are having mistake? Things the second one in a row where I’ve had issues. This time I bought an online order with prestige perfume etc but got an order today with my info/order slip but someone else’s skincare stuff. Like none of my order items were in there. They said they could refund my order but couldn’t send another one out because one item is out of stock then said I was screwed on the GWP as well. I’m super bummed but also really annoyed- they want me to take the stuff they sent me back to the store but couldn’t even help with the big missed GWP I wasn’t able to get?