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

CodeDisplayDefinition
first FirstFirst week of the month.
second SecondSecond week of the month.
third ThirdThird week of the month.
fourth FourthFourth week of the month.
last LastLast 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>