I got it!
DC worked, but I also did it in buckets of time which significantly improved the speed. Thank you all for your input.
The final query looks like this:
sourcetype=iis| bucket _time span=1mon | eval Month=strftime(_time,"%B") |eval MN=lower(Month) | eval sort_field=case(MN=="january",1, MN=="february",2, MN=="march",3, MN=="april",4, MN=="may",5, MN=="june",6, MN=="july",7, MN=="august",8, MN=="september",9, MN=="october",10, MN=="november",11, MN=="december",12) | stats dc(JSESSIONID) as Visits by Month, sort_field | sort +sort_field | fields - sort_field
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