Comcast's webmail does not offer a way to display the date in the message list. Today's messages show the time, messages from the previous 6 days show the day of the week, and older messages show the date.
If you want to see the date in the message list you'd need to use a program or app that provides what you're looking for. Their Xfinity Connect app, for instance, displays time in the message list for today's messages, and the date for older messages.
