First, you need a table with all those August dates in it. Do you already have one? If not, you can create a permanent one or use a CTE to make it on the fly. Second, cast the ResgistrationDate column as date in order to strip out the time portion. Third, left join your dates table to your registration table on dates.date < registration.date + 28 and patient = MR-02-0613-0001. Rows with NULL on the right hand side will be irregular; all others will be regular. Have a go at writing that and post back if you're struggling with something in particular.

John