onOpenCalendar(container) {
    
    container.navigateTo  = (event: any): void => {
      container._store.dispatch(container._actions.navigateStep(event.step));
      console.log(event);
      // you will get step as month -1 or +1 and based on that you can do your stuff
    }; 
  }

import { BsDatepickerInlineDirective } from 'ngx-bootstrap/datepicker';
import { BehaviorSubject } from 'rxjs';

interface BsDatePickerStoreView {
  date: Date,
  mode: keyof { 'day', 'month', 'year'}
}

interface BsDatePickerStoreData {
  view:BsDatePickerStoreView
}

export class BsDatePickerUtils
{
  public viewChanged = new BehaviorSubject<Date>(undefined);

  private lastKnownViewDate: Date = undefined;

  constructor(datePicker: BsDatepickerInlineDirective) {
    const store = (datePicker as any)._datepicker.instance._store.source as BehaviorSubject<BsDatePickerStoreData>;
    store.subscribe((data) => {
      const viewMode = data.view.mode;
      const date = data.view.date as Date;
      if (!this.lastKnownViewDate || this.lastKnownViewDate.toDateString() !== date.toDateString()) {
        if (data.view.mode === 'day') {
          this.lastKnownViewDate = date;
          this.viewChanged.next(data.view.date);
        }
      }
    });
  }
}

// Usage example
@Component({
  selector: 'my-component',
  template: `
<div>
  <bs-datepicker-inline #datePicker></<bs-datepicker-inline>
</div>
  `
})
export class MyComponent implements OnInit, AfterViewInit {

  @ViewChild(BsDatepickerInlineDirective) datePicker: BsDatepickerInlineDirective;
  bsDatePickerUtils: BsDatePickerUtils;

  ngAfterViewInit() {
    this.bsDatePickerUtils = new BsDatePickerUtils(this.datePicker);
    this.bsDatePickerUtils.viewChanged.subscribe(date => {
      console.log('Calendar changed: ' + date);
    });
  }
}

		const storeSubject = (datePicker as any)._datepicker.instance._store.source as BehaviorSubject<any>;
		storeSubject.subscribe(data => {
			const date = data.view.date as Date;
			if (
				!this.lastKnownViewDate ||
				(this.lastKnownViewDate.getMonth() !== date.getMonth())
			) {
				if (data.view.mode === 'day') {
					this.lastKnownViewDate = date;
					this.viewChangedSubject.next(true);
				}
			}
		});

<input (onShown)="onOpenCalendar($event)" bsDaterangepicker [bsConfig]="bsConfig">

interface BsDatePickerStoreView {
  date: Date,
  mode: keyof { 'day', 'month', 'year'}
}
interface BsDatePickerStoreData {
  view:BsDatePickerStoreView
}
export class Component {
  lastKnownViewDate: Date = undefined;

 onOpenCalendar(container) {
    const store = container._store.source as BehaviorSubject<BsDatePickerStoreData>;
    store.subscribe((data) => {
      const date = data.view.date as Date;
      if (!this.lastKnownViewDate || this.lastKnownViewDate.toDateString() !== date.toDateString()) {
        if (data.view.mode === 'day') {
          this.lastKnownViewDate = date;
          console.log(data, date);
          // do the on change operations
        }
      }
    });
  }
}