Вы можете избежать косой черты как %2F
.
Вот отредактированный пример:
updateEmployee(userName: string, holiday: Holiday): Observable<Employee> {
let parsedDate = Date.parse(holiday.date);
let month = parsedDate.getUTCMonth() + 1; //months from 1-12
let day = parsedDate.getUTCDate();
let year = parsedDate.getUTCFullYear();
return this.http.put(this._employeeUrl + '/holiday?userName=' + userName + '&Date=' + day + '%2F' + month + '%2F' + year + '&StateVal=' + holiday.state, '')
.map((res: Employee) => res)
.catch(this.handleError);
}