Giftwrap sold me calendars with wrong days

I bought 4000 calendars from Giftwrap. I proved the layout of the design on the calendars. <br> When the calendars came back the dates for May month where wrong e.g First day of the May month on their calendar starts on Saturday instead of Sunday. I was doing this calendars for the municipality and they had already distributed them to the community. There are only 400 boxes left. <br> Now Giftftwrap tells me that