My kids give me a card, but I'm still doing the same old things on mothering sunday, so it is not a vital event to me or my family - including my Mum.
If you want to celebrate it and cannot on the actual day (you could be working or it may be a weekend they spend with dad if separated), of course you can celebrate it on another day. If I'm down to work a shift on a special day, I just celebrate on another day, not everyone has the luxury of rearranging things to suit themselves.