I'd say, "I need to buy your Mum dinner", "I will treat you to dinner". 'Dinner' is often used to refer to the the main meal - midday or night ... it varies. The term is common in UK English, but those in the US aren't so familiar with it.

If it’s a formal occasion you would say a but for conversational English just dinner would do

In Us English they are slightly different. When you use “a dinner”, it means you will do it someday, maybe today, maybe later. Using “dinner” usually means you will do it on a specific day, very often today. (But it’s can mean the same as “a dinner.” “Your mom helped me over the past few weeks. I need to buy her (a) dinner.” “I’m glad we got the work done. Can I buy you dinner?”

A dinner is a formal occasion, for example at a wedding. A dinner party would be less formal, but is still an event. However, even if you are inviting someone to a formal occasion, you would still invite them to dinner, not to "a dinner." The fact that it is a formal occasion would be implied by the context.

So is it wrong to say:After one or two dinners out or glasses of wine, etc., I feel my dates are waiting for me to treat them. Use dinners?