*When a holiday falls on a Saturday, it is observed on the preceding day. (When the preceding day is also a holiday, both holidays are observed on preceding days.) When a holiday falls on a Sunday, it is observed on the following work day.
5.) Sehr geehrte Kunden, unser Büro ist vom 24 Dezember bis zum 2.Januar nicht besetzt. Sie erreichen uns wie gewohnt ab Montag den 5. Januar. Wir wünschen Ihnen und Ihrer Familie ein frohes Weihnachtsfest und ein gutes und erfolgreiches neue Jahr.

I often see people put public holiday notices in their email signatures a week or two in advance, especially where there are multiple affected dates in a row. We are a very date-dependent field, though.
What we need in our work communication is not more professional politeness or less formal, chat-based messaging applications like Slack. We need honesty. The problem is that we’ve conditioned ourselves to see honesty as self-indulgent or disrespectful. I’d argue the opposite is true. Honesty, even if it’s a bit more inconvenient for all parties in the moment, pays dividends later. It builds trust. When my partner Anne Helen Petersen and I were interviewing people for our forthcoming book on remote work, a frequent lament from both middle managers and workers was that they didn’t feel like they knew how to succeed in their jobs; that they were guessing what their superiors and coworkers wanted and, even when they asked, they didn’t quite trust the responses they got back.