{:x {:title "Week"
        :timeUnit "yearmonthdate"
        :field :WeekBeginning
        :axis {:format "%b"
               :labelAlign "left"
               :labelExpr "datum.label[0]"
               :timeUnit "month"
               :tickCount 12}}

Update: I ended up shifting the timeUnit of the lines to "yearmonthdate" and the timeUnit of the axis to "month" and was able to get it to format the way I wanted.