Here's an app you can connect with your slack channels and get events notifications from Google right there: Slack App
The above link contains the documentation and link to the app to connect that app to your Slack channel.
If you are using this app, you can change the reminder date right from your Google Calendar's event entry, e.g., if you want to get a reminder 30 minutes before the event starts, just do so on your event and you'll get a reminder 30 minutes before the event starts. Same goes for the a day before. Change the reminder setting of that event to a day before the event starts and it's done.