It's a traditional celebration so a traditional Sunday dinner is best, unless you know your mum has a favourite meal that you can cook.
If there are not so many of you I would recommend chicken - a large one, say 2.5 kg will feed up to 6 people and it's easy to prepare in advance and then just put in the oven. A classic roast chicken with lots of herbs, garlic and butter is the easiest, and you can find an easy but tasty mother's day recipe here.
You could do two chickens for a large party but then it's probably easier to get a joint. Lamb is very nice just at this time of year and a typical spring dish - you could get a large joint and roast it with rosemary, or do a more elaborate dish maybe with red wine.
For starters and desserts you can find lots of special recipes here.