What we learned from preparing for a Linked Open Data hackathon Experiences from a first timer team of the KB, national library of the Netherlands Olaf Janssen – olaf.janssen@kb.nl – @ookgezellig – 0000-0002-9058-9941, Mirjam Cuper - Mirjam.Cuper@kb.nl – @CuperMirjam – 0000-0003-0187-9873, Thomas Haighton –Thomas.Haighton@KB.nl – @ThomasHaighton, Martijn Pluim – Martijn.Pluim@KB.nl, Roosmarijn de Groot – Roosmarijn.deGroot@KB.nl – @RoosmarijndeG, Daniëlle Jansen - Danielle.Jansen@KB.nl – @ecritures, Koninklijke Bibliotheek, national library of the Netherlands Lotte van den Eertwegh - lotte.van.den.eertwegh@outlook.com, Dutch Ministry of Internal Affairs The HackaLOD1 is an annual one day hackathon of the heritage field in the Netherlands focusing on linked open data (LOD). Teams of programmers, historians, designers, and other heritage and data experts are challenged to create new, creative applications with collection data from heritage institutions, resulting in new insights into Dutch heritage by connecting collections. For the 2020 edition we formed a KB2 team to participate for the first time, bringing together colleagues from various departments, whose knowledge about SPARQL3, Wikidata4 and other LOD technologies ranged from basic to intermediate. As we never participated in a hackathon before and we are no LOD gurus who could hack together an application within a single day, we needed to make the necessary preparations well in advance. Due to COVID19 regulations the 2020 HackaLOD was repeatedly postponed, now to November 2022, but that did not discourage us from continuing our preparations, as team based learning from these preparations was a goal in itself. First, we discussed the boundary conditions for our project. With the KB collection as our natural starting point, a requirement for the HackaLOD was to reuse LOD from other institutions as well. Furthermore, we wanted to link the past to the present, bringing heritage into a 21st century context. Finally, inspired by the success of KB’s Medieval Meme Generator5, we wanted to build a fun and not-too-serious application. We aimed for collection objects that allowed us to build our own rich LOD cloud, not only by interlinking entities (people, locations, datetimes, events etc.) within our own datasets, but especially by connecting to entities in LOD endpoints of other parties as well. We found the Album amicorum of Jacob Heyblocq6 [abbrev. AAJH] to be an excellent starting point. This 17th century book of friendship7 was created by Jacob Heyblocq (1623-1690), poet and headmaster of the Amsterdam Grammar School8. It holds contributions by Rembrandt9, Vondel10, Huygens11, Cats12 and 137 other persons from the Dutch Golden Age13. This allowed us to make semantic links between Heyblocq and all album contributors. As many of these contributors are described in LOD databases of other parties, it allowed us to retrieve additional information from those sources. Furthermore, most contributions are signed with both a placename (city) and a date, allowing us to make connections between persons, locations and dates. Finally, as some contributors maintained alba themselves, it should theoretically be possible to make connections to those alba and their contributors, contributions, locations and dates, adding even more nodes to our AAJH-LOD- cloud. To build this cloud, we used Wikidata as our storage and created Wikidata items for the album itself14, for all contributions15 and for all contributors16. We did this by manual input via the Wikidata web interface and by semi-automated input via OpenRefine17, adding batches of data from local Excel sheets. We made sure all the entities were properly interconnected. This way, we were able to request data directly linked to the contributors, such as their genders, places and dates of birth and