Just curious

Having a separate wedding celebration is totally okay! As long as your guest know they are already married. It is never appropriate to ask for gifts, monetary or otherwise. They can have a registry and pass the info along by word of mouth or on a wedding website.

Just curious why 2 extra celebrations? I think most people usually only have 1. Just hoping the reasoning won't seem gift grabby.
But otherwise it's totally fine. They're usually called celebrations of marriage if they arent same day of the actual wedding ceremony.
And like PP said, asking for gifts is always a no no, just have her register (or not if they prefer cash gifts) and people should hopefully know what that means!

Lots of people have a small ceremony, and a large-group reception at a later date. My parents did decades ago, & they're even more popular now. There's a few basic rules.

1. After the small ceremony, you must have a small reception for the attendees. It needn't be expensive or fancy, but it must be done. At home, or a restaurant, wherever. The people who came to your wedding deserve to be thanked for attending & invited to celebrate with you. Feed them.

2. Make your invites clear. Apart from the 20, you're essentially inviting people to celebrate a marriage that has already taken place. It's a post-elopement party! This is totally fine, just make sure your wording reflects that, so people don't think they're showing up for a wedding.

3. It's never appropriate to ask for gifts. If people wish to give, they may, but it isn't required (even at traditional weddings, FYI).

Other things you haven't asked but might like to know:

4. Don't re-enact the wedding or have the couple pretend they're not married, that's weird.

5. Everything else can basically be as similar to or different from a normal reception as you like. Also since some people seem confused on this point: the bride can absolutely wear her wedding dress to a later-date reception if so desired. She doesn't have to, but there's no rule against (Yes, really, women used to rewear their wedding dresses all the time).

I had a destination wedding, so only immediate family members came. My mom had a nice cookout at her house afterwards and we had sent out invitations with some catchy phrase we found online. It basically told people that we got married and my mom was hosting a celebration of the marriage. We had a wedding cake under a tent and Cut the Cake and served it ( but that's as traditional as it got). It seemed more like a family reunion. We did receive a few gifts from close family members, but I definitely didn't put on the invitations that I was registered anywhere. Even though I didn't get many presents, I saved a fortune on not having a big fancy wedding and could put that money towards whatever I wanted.
Good luck and keep in mind that people will inquire as to why they weren't invited to the wedding so if you plan to invite them to the Celebration, be prepared to answer those type of questions.