Hi there
Attending an open day is a great idea!
You're right in saying that the teaching for these courses happens on the science site/lower Mountjoy. For Econ/Finance, you may also need to go to the Business School sometimes which is about a 12-15 minute walk from the science site.
Because Durham's quite small, all colleges are a fairly short distance from the science site. But the hill colleges are located closer to it than the bailey ones so they might be more accessible for you. If you go to the business school, you'll probably see all of them on your way to it. Trevs, Grey, Marys, Collingwood and Van Milert are the five hill colleges that are catered and have mostly single rooms. Ensuites are mostly offered in the self-catered hill colleges out of which South and John Snow are almost next door to the business school and about 12 mins walk away from the science site.
For Marys, Mildert and Trevs, you're allowed to stay in during Christmas and Easter breaks no matter what room you're in. For Collingwood, you only have to move out during these periods if you're in an ensuite room (although some belongings can be left in the under-bed storage). Grey offers both short and standard lets and those on short lets must vacate their rooms in the Easter vacation as Grey College trades at this time. Although I think all undergraduates are usually offered standard lets only so it shouldn't be a problem.
Depending on what else you're planning to see (like the department buildings, course presentation, sports facilities at Maiden Castle, etc), I think you might have sufficient time to tour max 3-4 colleges and see the rooms (if possible) at maybe 1-2 of them. You will also be able to see other colleges when you walk past them. I would suggest looking at a room in Trevs as they're shaped differently due to the college's pentagonal structure, and one of the single standard rooms at any of the other catered colleges. If you're seeing a self-catered college, check out either South or John Snow as they have quite similar ensuites and flat systems.
Here's also some top tips from our student ambassadors on attending an offer holder day but it runs quite similarly to an open day so worth looking at:
https://www.tiktok.com/@thedurhamstudent/video/7349996266165669152Hope that's helpful, let me know if you have more questions

-Himieka