cal, err := ParseCalendar(strings.NewReader(input))

  cal := ics.NewCalendar()
  cal.SetMethod(ics.MethodRequest)
  event := cal.AddEvent(fmt.Sprintf("id@domain", p.SessionKey.IntID()))
  event.SetCreatedTime(time.Now())
  event.SetDtStampTime(time.Now())
  event.SetModifiedAt(time.Now())
  event.SetStartAt(time.Now())
  event.SetEndAt(time.Now())
  event.SetSummary("Summary")
  event.SetLocation("Address")
  event.SetDescription("Description")
  event.SetURL("https://URL/")
  event.AddRrule(fmt.Sprintf("FREQ=YEARLY;BYMONTH=%d;BYMONTHDAY=%d", time.Now().Month(), time.Now().Day()))
  event.SetOrganizer("sender@domain", ics.WithCN("This Machine"))
  event.AddAttendee("reciever or participant", ics.CalendarUserTypeIndividual, ics.ParticipationStatusNeedsAction, ics.ParticipationRoleReqParticipant, ics.WithRSVP(true))
  return cal.Serialize()