The 3rd phase of the Centre of Excellence (CoE) in Simulation of Weather and Climate in Europe, ESiWACE3, presents its first hackathon, “Running weather and climate models on LUMI”. The event aims at helping climate scientists port and optimise their codes to new architectures, such as LUMI, and will be the first of four hackathons organised through the project. It will occur on 18-20 October 2023 at the CSC-Tieteen Tietotekniikan Keskus Oy (CSC) premises in Espoo (Finland). It will be an on-site event―with no remote participation possibility―and no attendance fee.