Currently in Thunderbird, when the Mail tab is viewed, the New Event button in the Today pane defaults to creating a new event starting at the next full hour on the current date. Alternatively, if the Calendar tab is open, the New Event button defaults to whatever date is selected there, even if the Mail tab is being viewed
This behaviour can be changed by going into Settings > Calendar > Events and Tasks and setting Start Date and Due Date. There are currently no options that are responsive to the interface.
Suggestion: When the Today pane is set to display mini-month, the New Event button should detect the date selected on that calendar and default to that date when clicked to create a new event i.e. if 6 Oct is selected, clicking New Event should open a dialog with Start and End dates set to 6 Oct.
Rationale: This feels more intuitive, as the New Event button sits directly underneath the mini-month in the Today pane, and directly above the agenda of events for any selected date.