For details about what BSP Marathon is see d-Devel-Announce. It might be also a good idea to have read the HostingBSP howto. It gives some ideas what background work is needed to host a BSP.
Some useful links:
This page documents the Etch release BSP marathon. For Lenny, refer to BSPMarathonLenny.
Past BSPs (2006)
09-11 June
École Centrale Paris: BSP/2006/06/fr/Paris on June 9 to 11, 2006
11-13 August
Debian-Edu/Skolelinux german testcenter in Gütersloh
DebianMéxico (spanish) http://www.red-libre.org/pipermail/debianmexico/2006-August/002083.html in Mexico City.
08-10 September
Don Bosco Studentenheim, Vienna BSP/2006/09/at/Vienna
AILab, University of Zürich (one-day Saturday only): BSP/2006/10/ch/Zurich
15-17 September
Jülich, Germany, credativ GmbH, BSP/2006/09/de/Julich
16 September
San Cristobal, Venezula, BSPVenezuela
22-24 September
[canceled] Berlin, Germany BSP/2006/09/de/Berlin (BSP has been canceled due lack of participants)
29 September -- 01 October
Utrecht, the Netherlands, BSP_Utrecht_2006
06-08 October
Espace Autogéré des Tanneries, Dijon, France: TanneriesBSP
AILab, University of Zürich: BSP/2006/10/ch/Zurich
13-15 October
München, Germany: BSP/2006/10/de/Munich
18 October
Utrecht, the Netherlands BSP_Utrecht_2006 (Bug Squashing Afterparty)
28-29 October
Cambridge, UK BSP/2006/10/gb/Cambridge
11-12 November
Helsinki, Finland BSP/2006/11/fi/Helsinki
28 December
- Mexico City, Mexico.