Ion select option Filter

I’m trying to do a filter that gets some data based on the month.
I’m using select options. I want to get the value only if the user chooses one of the select options if the user doesn’t tap the filter at all then show them all. Like for the buttons is the (click) option.
What can I do for the select options?

My code

<ion-item style="background-color: blueviolet;margin: 0px 0px 0px -17px;">
      <ion-label style="font-size: 16px;font-family: 'Poppins', sans-serif; ">Filter by Month</ion-label>
      <ion-select (click)="getMonth()" [(ngModel)]="getSelectedSubject" style="font-size: 16px;font-family: 'Poppins', sans-serif;">
        <ion-select-option value="January">January</ion-select-option>
        <ion-select-option value="February">February</ion-select-option>
        <ion-select-option value="March">March</ion-select-option>
        <ion-select-option value="April">April</ion-select-option>
        <ion-select-option value="May">May</ion-select-option>
        <ion-select-option value="June">June</ion-select-option>
        <ion-select-option value="July">July</ion-select-option>
        <ion-select-option value="August">August</ion-select-option>
        <ion-select-option value="September">September</ion-select-option>
        <ion-select-option value="October">October</ion-select-option>
        <ion-select-option value="November">November</ion-select-option>
        <ion-select-option value="December">December</ion-select-option>
      </ion-select>
    </ion-item>

.ts file

ngOnInit() {
      this.username = this.cartservice.username;
        this.api.getEmployeeIDByUsername(this.username).subscribe(res=>{
          console.log(res)
          this.employeeid = res[0].Staff_ID;
          this.getALLPdfDataID(this.employeeid);
        });
    }

    getByMonth(){
      this.api.getEmployeeIDByUsername(this.username).subscribe(res=>{
        console.log(res)
        this.employeeid = res[0].Staff_ID;
        this.getALLPdfDataIDMonth(this.employeeid,this.getSelectedSubject);
      });
    }

    getALLPdfDataID(item){
      this.api.getpayrollfilesId(item).subscribe(res=>{
      console.log(res)
      this.pdfData = res;
      })
    }

    getALLPdfDataIDMonth(item,month){
      this.api.getpayrollfilesIdMonth(item,month).subscribe(res=>{
        console.log(res)
        this.pdfData = res;
      })
    }

Update

html

<ion-select (ionChange)="onContextChange($event)" [(ngModel)]="context" style="font-size: 16px;font-family: 'Poppins', sans-serif;"> 

.ts file

onContextChange(ctxt: string): void {
      if(this.context ==="All"){
        this.getALLPdfDataID(this.employeeid);
      }
      else{
      this.api.getEmployeeIDByUsername(this.username).subscribe(res=>{
        console.log(res)
        this.employeeid = res[0].Staff_ID;
        this.getALLPdfDataIDMonth(this.employeeid,this.context);
      });
    }
    }