We are closing for two days for renovation. Can I tell all my customers on WhatsApp?

Yes, with a campaign. Open Marketing → Campaigns, press New campaign, write the message or start from a template, and choose the audience Everyone who has ordered. Pick WhatsApp as the channel and send. It goes out from your own number one message at a time, inside the daytime sending window, so a campaign sent at eleven at night starts after ten the next morning. It counts against your weekly cap per customer, which you set under Review queue & weekly cap. Also tick Closed on those days under Settings → Working hours, so anyone who opens your page sees the closed screen with the date you reopen. Restaurant marketing software here is the same screen as your promotions.