This code system http://hl7.org/fhir/week-of-month defines the following codes:

Code Display Definition
first First First week of the month.
second Second Second week of the month.
third Third Third week of the month.
fourth Fourth Fourth week of the month.
last Last Last week of the month.
<status value="active"/> <experimental value="false"/> <date value="2022-06-14"/> <publisher value="HL7 (FHIR Project)"/> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <description value="The set of weeks with in a month."/> <jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <caseSensitive value="true"/> <valueSet value="http://hl7.org/fhir/ValueSet/week-of-month"/> <content value="complete"/> <concept> <code value="first"/> <display value="First"/> <definition value="First week of the month."/> </concept> <concept> <code value="second"/> <display value="Second"/> <definition value="Second week of the month."/> </concept> <concept> <code value="third"/> <display value="Third"/> <definition value="Third week of the month."/> </concept> <concept> <code value="fourth"/> <display value="Fourth"/> <definition value="Fourth week of the month."/> </concept> <concept> <code value="last"/> <display value="Last"/> <definition value="Last week of the month."/> </concept> </CodeSystem>