The date you should RSVP by will be indicated on your invitation.
If you're the kind of person who likes clearly outlined expectations: semi-formal to dressy casual. For us this means, essentially, "wear what you'd normally wear to a wedding" (I, Katie, am directly quoting Mark here). For those of you who appreciate nuance and creative freedom: The main thing we care about is that you feel comfortable in what you're wearing, so wear an outfit you think you look good in! Who cares what "dressy casual" means. Our one note is that it will be the beginning of May in Minnesota, so we recommend layering so you can be prepared for a variety different weather patterns (and checking your favorite weather app in the days before). I'm looking at you, my dear Californian friends.
Unfortunately we're working with a set limit on the number of guests we can invite to our wedding, and as such, we're only inviting children who will have a role in our wedding. If your children are invited, it will be clearly indicated on your invitation. That said, if you do or will have a newborn by the time of our wedding who will need to come in order for you to come, please send us an email and let us know!
As we mentioned above, the limit on our guest list is pretty strict. If you were given a plus one, it will be reflected on your invitation. If your invitation doesn't reflect this and you'd like to bring a plus one, let us know and we'll see what we can do about it!