Very intense, and on their time - you will be asked to jump for them, and become 'cultured' into their values, yet they disrespect candidates left right and centre. You will be bullied in the interview, and if you do not provide references from managers (even if you have never had a manager) then they won't even look at you - the YMCAs human resources policies are so outdated as a reference is nothing today (it means nothing if you provide a managers reference from 10 years ago...and ya, they force you to give them 3 management references).
Business professional wear especially if you are interviewing for one of the administrative positions. Something like a suit or suit jacket. If you are interviewing for one of the community support positions, business casual. Something like slacks or a skirt. No jeans.