Thank you for this, Jessica!Depends on the context. If you are saying something like “I attended an open day with Joe Bloggs LLP” then lower case. If it is the title of the event, eg “I attended the Innovation Open Day” then it is typically capitalised, although look at how the firm writes it in things that are not headings (eg how they write it in the copy of their website/brochure), and mirror the way they write it.