The. It was a specific meeting.
Interested in Language
Which should I use:
With reference to the meeting we had on 1 Jan or With reference to a meeting we had on 1 Jan.
If you're writing to another person who was at the meeting with you, you would say either "With reference to the meeting we had on 1 Jan, ..." or "With reference to one of the meetings we had on 1 Jan, ...".
If you were writing to someone who wasn't at the meeting, you might write "With reference to a meeting I had on 1 Jan, ...".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.