I need to mend eight pieces of clothing....

[thehammer]

Is there any difference between 'one day' and 'a day' in the following sentences?

1- I need to mend eight pieces of clothing in one day.

2- I need to mend eight pieces of clothing in a day

 

[Tarheel]

Not to me.

 

[Barque]

Is there any difference between 'one day' and 'a day' in the following sentences?

Are you trying to emphasise that all those pieces have to be mended in a day? Use the second. And you need a full stop at the end.

Are you saying you have to do it that day? Use "today".