The list of features from Windows Phone that needs porting over to Windows 10 Mobile is long, of course - or at least it was long, more and more keep showing up in core application updates. In this case, Windows Phone's Calendar's collapsible 'Week view' is now fully implemented in Windows 10 Mobile's Outlook Calendar. (In fairness, there's an equally long list of features in Windows 10 Mobile and its core applications that will never make it back to Windows Phone 8.1, so overall everyone really will be better off with '10'... honest!)
Previously, Outlook Calendar was limited to 'Agenda' and 'Day' views, plus the month view that's available from the top bar pick list. But Week view, one of the most popular features of Windows Phone 8.1 Calendar, was missing. With the update to 'Outlook Mail & Calendar' that's rolling out now, Week view is implemented more or less exactly as it was under 8.1 - tap on a day to expand its contents within the week 'grid':
The new view, with one day at a time 'expanded' - tap any other to see its contents and contract the previously expanded one, etc. Tapping on the new bottom toolbar 'View' control is the way into the new view (the control was previously just a toggle between the two existing views)...
A nice little tweak, and notice that the view now works with Windows 10 Mobile set to a 'dark' theme - though if you tap on an entry, everything turns white again - I do hope they manage to keep everything dark (or theme aware) eventually.
Check Store on your Windows 10 Mobile Insiders Preview device to see if the update has reached you yet, etc.