Regardless of what should be possible, earlier it wasn't possible to login - atleast on US servers. I can validate the OPs problem because I too was trying to login to change my data stuff around following the recent changes to it.
I don't know exactly when (as I don't generally log in over the weekend) but they changed and now allow users to log in not only into the website but onto TWS as well 24/7. I posted a screenshot earlier from my mobile but the Java app works equally well Saturdays and Sundays. Obviously you cant trade, whether you can pull historical data for charts is something I can't confirm right now.
I can confirm it doesnt work. On weekends they probably have some maintenances so it only works like 50% of the time
Saturday mornings typically has issues logging in, the last few times I tried. Probably system maintenance. Usually works by 12, so you can check if you were assigned anything.