class Event{
    public Date when;
    public String eventName;
}

repository.getAllEvents()
            .toObservable()
            .flatMap(new Function<Events, Observable<Event>>() {
                @Override
                public Observable<Event> apply(@NonNull Events events) throws Exception {
                    return Observable.fromIterable(events.getEvents());
                }
            })
            .groupBy(new Function<Event, Date>() {
                @Override
                public Date apply(@NonNull Event event) throws Exception {
                    return event.getStartsAt();
                }
            })
    .flatMap(new Function<GroupedObservable<Date, Event>, Observable<Object>>() {
        @Override
        public Observable<Object> apply(@NonNull GroupedObservable<Date, Event> dateEventGroupedObservable) throws Exception {
            final Date key = dateEventGroupedObservable.getKey();
            return dateEventGroupedObservable.toList().toObservable().flatMap(new Function<List<Event>, ObservableSource<?>>() {
                @Override
                public ObservableSource<?> apply(@NonNull List<Event> events) throws Exception {
                    return Observable.just(new Pair<Date, List<Event>>(key, events));
                }
            });
        }
    })
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribeWith(new Observer<Object>() {
        @Override
        public void onSubscribe(Disposable d) {

        }

        @Override
        public void onNext(Object o) {

        }

        @Override
        public void onError(Throwable e) {

        }

        @Override
        public void onComplete() {

        }
    });