It's possible it was an oversight, or the invitation was lost. But it's best to at least acknowledge the possibility that you are actually not invited there.
So it looks like there are three possibilities:
- It was an honest mistake on the part of the person making up the invitations. But most people charged to do this are very careful about it, and the invitation list is usually checked by others as well. But it's possible that someone just fell through the cracks.
- The invitation was lost in transit. It happens, but it's uncommon.
- You didn't get the invitation because you were not invited. As saddening as this may be, and as uncommon as it may appear, this too is at least possible (though perhaps improbable).
OK, so if you did not receive an invitation, please do not just show up to the event. Only consider going if you get some form of invitation: in writing by the host (better late than never), an after-the-fact last minute invitation over the phone, or as the partner to someone who was invited.
So how do you gently remind someone that maybe you were not put on the invitation list by accident? This has happened to me on two occasions. Each time I felt somewhat hurt. The first time, many sad thoughts ran through my mind, but I let it go. I gave it thought however. About a year later, it happened again, and I did the following two actions.
- If it's a celebration (a birthday, graduation, marriage), write a congratulatory note to the happy person. Make no mention of not getting an invitation. Don't ask a question or write anything that would/should get a reply (that could also be construed as putting them on the spot). Aside from the actual congratulatory message, it will serve as a reminder to the person.
Hopefully, it will get them to ask themselves why they are being congratulated when they are coming to the event. It may well lead to a correction. But you should be prepared to accept that it does not change anything.
- When talking to a mutual friend who is invited, work into the conversation that you will be doing something at the same time as the celebration. It may be difficult, but try not to be obvious about it. The hope is to get a question like "Aren't you going to so-and-so's party?" Then you can say that you did not get an invitation. This should cue your friend to inquire on their own about this situation.
By the way, I suggest being honest in that if you said you plan on doing something, then you actually need to be planning on doing it. That was the case for me. I got a sympathetic look from the friend. That told me all I needed to know. I tried to enjoy my solitary activity.
Best of luck in getting a clarification on whether you should have been invited.