This is the 2560th most frequent Russian word.
случаю
Dative case of "случай," meaning "case" or "occasion" in English.
The word "случаю" in this sentence is used as part of the phrase "по случаю", which translates to "on the occasion of" and is employed to indicate that something is done as a recognition or celebration of a specific event.
По случаю праздника мы организовали вечеринку.
On the occasion of the holiday, we organized a party.
Here, "случаю" appears in the phrase "в случае", meaning "in case of" and is used to specify a conditional situation that could affect the context.
В случае дождя мероприятие будет перенесено.
In case of rain, the event will be rescheduled.
In this sentence, "случаю" implies "on a special occasion" where it refers to a specific event or circumstance that provided an opportunity for the action.