You can do this by creating a dynamic group, which contains recipients who subscribed on the same day (‘All recipients’ –> ‘Search recipients’–> ‘created on’ –> ‘dynamic search criterion’ –> ‘1 day after’ –> ‘Save recipient to group’). Following this, create a new newsletter and select the group you just created during step 4.  Then select ‘Send regularly’ during step 5 choose the frequency of daily, and select a time for the next day. This will ensure that all recipients who sign up on a given day will receive a newsletter on the same day – at the time you selected.