I have always used "in the weekend" for future tense and "at the weekend" for past. I have noticed lately the use of "on the weekend" which I don't think sounds quite right. Someone told me that they think that's American to say 'I did this on the weekend'. Does anyone know what is correct or if it really matters?
In British English we use 'at' and in American English 'on'. I wouldn't ever use 'in the weekend':
I saw him at the weekend.
I'll see her at the weekend.