occasion
/əˈkeɪʒən/
The noun »occasion« covers both a special event or celebration and a specific point in time or instance when something happens. The right German equivalent depends on whether the focus is on the event itself or on the particular moment.
1. Anlass
When »occasion« refers to a special event, celebration, or the reason behind something.
A wedding is always a joyful occasion.
Eine Hochzeit ist immer ein freudiger Anlass.
The museum was reopened on the occasion of its centenary.
Das Museum wurde anlässlich seines hundertjährigen Bestehens wiedereröffnet.
She kept the dress for a formal occasion.
Sie hob das Kleid für einen feierlichen Anlass auf.
2. Gelegenheit
When »occasion« denotes a specific instance or moment in time when something occurred.
I have met her on several occasions.
Ich bin ihr bei mehreren Gelegenheiten begegnet.
On this occasion, the negotiations proved more difficult than expected.
Bei dieser Gelegenheit verliefen die Verhandlungen schwieriger als erwartet.
Notes
German »Gelegenheit« covers both »occasion« (a specific instance) and »opportunity« (a chance to do something), which are distinct words in English. The phrase »on the occasion of« often translates more naturally as the preposition »anlässlich« rather than a full noun phrase.