The work-related costs scheme (WKR) is a tax scheme that allows employers to reimburse certain costs to staff untaxed. This also applies to company anniversaries.
The big advantage: events that take place at your own company site fall fully under the WKR. The costs for catering, entertainment and production are thereby largely untaxed. For a large anniversary that can save thousands of euros.
Different rules apply to external venues. You can still place part of the costs under the WKR, but it is more limited. Always consult your accountant or tax adviser for the exact options.
Tip: combine your anniversary with a content-driven programme (think of an annual kick-off or strategy session). Then you can account for part of the costs as a business meeting.