package net.squanchy.eventdetails;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.widget.Toast;

import net.squanchy.R;

import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;

public class EventDetailsActivity extends AppCompatActivity {

    private static final String EXTRA_EVENT_ID = "event_id";
    private static final String EXTRA_DAY = "day";

    private EventDetailsService service;
    private Disposable subscription;

    public static Intent createIntent(Context context, int dayId, int eventId) {
        Intent intent = new Intent(context, EventDetailsActivity.class);
        intent.putExtra(EXTRA_DAY, dayId);
        intent.putExtra(EXTRA_EVENT_ID, eventId);
        return intent;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_event_details);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        EventDetailsComponent component = EventDetailsInjector.obtain(this);
        service = component.service();
    }

    @Override
    protected void onStart() {
        super.onStart();

        Intent intent = getIntent();
        int dayId = intent.getIntExtra(EXTRA_DAY, -1);
        int eventId = intent.getIntExtra(EXTRA_EVENT_ID, -1);
        subscription = service.event(dayId, eventId)
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(event -> Toast.makeText(this, "Event: " + event, Toast.LENGTH_LONG).show());
    }

    @Override
    protected void onStop() {
        super.onStop();

        subscription.dispose();
    }
}

package net.squanchy.eventdetails;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.widget.Toast;

import net.squanchy.R;

import io.reactivex.android.schedulers.AndroidSchedulers;
import io.reactivex.disposables.Disposable;

public class EventDetailsActivity extends AppCompatActivity {

    private static final String EXTRA_EVENT_ID = "event_id";
    private static final String EXTRA_DAY = "day";

    private EventDetailsService service;
    private Disposable subscription;

    public static Intent createIntent(Context context, int dayId, long eventId) {
        Intent intent = new Intent(context, EventDetailsActivity.class);
        intent.putExtra(EXTRA_DAY, dayId);
        intent.putExtra(EXTRA_EVENT_ID, eventId);
        return intent;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_event_details);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        EventDetailsComponent component = EventDetailsInjector.obtain(this);
        service = component.service();
    }

    @Override
    protected void onStart() {
        super.onStart();

        Intent intent = getIntent();
        int dayId = intent.getIntExtra(EXTRA_DAY, -1);
        int eventId = intent.getIntExtra(EXTRA_EVENT_ID, -1);
        subscription = service.event(dayId, eventId)
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(event -> Toast.makeText(this, "Event: " + event, Toast.LENGTH_LONG).show());
    }

    @Override
    protected void onStop() {
        super.onStop();

        subscription.dispose();
    }
}

Use long for eventId when creating EventDetailsActivity Intent