You are forgetting that sieges include multiple stages, not only Sunday when you attack castle, but also, bidding, declaration, etc. Moving one, will require moving the rest.
You see it as something small, but in fact it includes many parts, processes and actions. It has already been confirmed that sieges will not be moved any other day or time.
You also need to remember that it's not possible to find time that will be good for everybody. Maybe changed time will suit you, but that change will make it uncomfortable for many other players. There is never a perfect time.
I mean, I'm NOT forgetting that bidding, declaration, etc. are involved with the siege portion of the castle and honestly was assuming moving it back one day would cause all that to be moved as well. And while it isn't possible to find a time that is good for everybody, a majority of the world most likely has a typical Monday-Friday job or Monday-Friday schooling, with everyone wanting to enjoy themselves on a Friday or Saturday night; in some instances having nothing planned for Sunday since most schools/universities aren't open on Sundays which would allow players to sleep in after a fun-filled Saturday night. I'm from an NA timezone and even for me, Sunday sieges at the current day and time cut into the end-of-weekend crunch people typically have before they go to work the next day. It's late-as-fuck-o-clock at night for EU players on Sunday evenings (really, Monday mornings) when sieges happen, which cuts out a good portion of the EU player base.
There might not be a perfect time, but there damn sure can be a better day.