I am making a timeline for a story set in Tolkien’s Middle Earth and have defined four eras, one for each Age (First, Second, Third, Fourth). How is the weekdays calculated for eras? Can I define in some way that the e.g. 7th May 1251 Third Age is a Wednesday?
That depends on how you have set up your calendar. With AT2, the “AD” era begins on Monday. As we know, this is the last era which must have an “infinite” duration. If you insert a custom era without leap days before it, the custom era’s start date shifts by 365 days each year of duration, and the start weekday by one day each year.
So if your “Middle Earth” time count is in a self-defined era, you can move the desired day of the week by extending or shortening this era by the corresponding number of years.
This corresponds to my investigations with Aeon Timeline 2. Does it also apply to AT 3?
I think it makes sense to assume it works the same n AT3, I’ve not tested but I see two solutions: One to have an fake era with a number of years (I haven’t tried to see if an era can be fractions of a year, presumedly not) as you propose, or simply adopt to Aeon’s calculations since the weekdays aren’t important to the story.