The Delay until a Date flow allows you to delay processing of the next node in the Journey until a specified date has been reached. See Fig 1 below for an example where the Entrant event is a contact being added to a database, upon which the Delay until a Date Flow node will delay until a certain date before the Journey progresses to the Action, Send an Email.

Fig 1

To select the date, click the Delay until a Date flow node once you have added it to the Journey work area. Doing so will bring up this popup ("Properties") on the right-hand side of the screen.

Clicking the "Date" input field below the "Delay until a Date" label will allow you to select the date by clicking the small calendar icon in the far right of the input field.  (See Fig 2 below):  

Fig 2

Once you have finished with the action's setup, click the "Save Properties" button.