My MIL turned 90 at the beginning of 2020 and my SIL planned a family dinner, and an afternoon tea. The afternoon tea was my idea, but she planned it all (she's very good at planning), and I helped. I wasn't in favour of a surprise, but hey, her mum, her plan. I went along with it. We had it at a hotel that could seat around 50 if needed, and we did!
I must admit it all went very well. My MIL was surprised, but very pleasantly so. For the afternoon party, we did an open house; people came from her old neighbourhood, the church, bowling club, etc. There were quite a few that she hadn't seen for a while, and she had a really nice time visiting with everyone.
At the family dinner my SIL hired a photographer and we had some really nice photos, good memories now since my MIL's health deteriorated late last year and she passed away early this year.
I agree that having it on Mothering Sunday wouldn't be a good idea, and leaving it to be a surprise would be based on your opinion of how she would like it. Maybe telling her after it is already planned and give her a few day's notice.
Do you still wear you original wedding and engagement ring