Hope this will help:
"your query that includes _time field" | eval weeknumber=strftime(_time,"%V") | eval day = strftime(_time,"%d-%b-%y") | eval dt = strftime(strptime(strftime(_time,"01-%m-%Y"),"%d-%m-%Y"),"%V") | eval weeknumber = if(strftime(_time,"%m") == "12" AND weeknumber = "01","53",weeknumber) | eval weeknumber = "Week - ".(weeknumber-dt+1)
%v will give you Mon-Sun week and %u will give you Sun-Sat week - whatever you need. I've left extra fields in for explanation sake. You can filter them out.
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