Proceed as follows:

Create a dynamic group which contains all recipients whose birthday it is today (‘All recipients’ –> ‘Search recipients’–> ‘Date of Birth’ –> ‘Use dynamic search criterion’ –> ‘Every 1 year after birthday’ -> ‘Search’ -> ‘Actions for selected recipients’ -> ‘Save to group’ (make sure to check the box for ‘Save as dynamic group’ here!)).

When creating the newsletter, choose this group in step 4, ‘Recipients’ and in step 5 select ‘Send regularly’.

Enter the frequency of ‘daily’ and select the desired time. At this time every day, all those who have birthdays will find themselves in this group and will receive the birthday newsletter.