The Date Header is that part of a Blogger blog which displays the date in which a post was created. This is sometimes referred to as the date stamp as well. We can easily control whether the Date Header should be displayed or not, and if displayed, in what format should it be displayed.
First let’s see how the appearance of the Date Header can be controlled. In Layout -> Page Elements view, if you open the Blog Posts widget in Edit mode, you get the Configure Blog Posts dialog. In that, the first check box under Selected Items is the button to ON/OFF the display of the Date Header. This is illustrated by the first red arrow in the figure below.
The format of the date stamp can be controlled from the drop down next to the check box mentioned above. However, it can also be set from the Settings -> Formatting section as shown below.
The Date Header is displayed only once for a given date. i.e., if you do more than one post per day, all of them will get only one date stamp when they are displayed on the blog’s main page. (Each of them, however, will have a date header on their individual post pages.) This is because the layout data tag (data tags are bits and pieces of information held in the Blogger database with respect to each blog) for the Date Header is set only once for all the posts in a given day.
This data tag is the post.dateHeader tag and this Blogger Help page says that it is “only present if this is the first post in the list that was posted on this day”.