The loss of the labeling for the days of the week in the timeline is a most unfortunate view and in terms of usability a step backwards from the past. While I always know the days I don't typically know the dates and I suspect most people are the same. For example, If I want to look at the weekend, I want to find Sat/Sun and not a specific date.
It looks like this crucial semantic information was lost as part of making the 15 day forecast fit without sliders. However, I'd rather have sliders and have an easy-- and confident-- way to be sure of which day I'm looking at.
(The point about confidence is an important one. It's very easy to mistake numbers for each other, but much harder to mistake characters.)
Perhaps one way to resolve this is to put a single character for the day of the week, e.g. in the screenshot T 4, W 5, etc...
It's not a problem that Sat and Sun have the same letters, as they're still easily identifiable. Likewise for Tue and Thu.