Breakdown of Demà la reunió començarà a les set.
Questions & Answers about Demà la reunió començarà a les set.
Putting demà first is a very natural way to set the time frame right away: Tomorrow, the meeting will start at seven.
Catalan word order is fairly flexible here. You could also say:
- La reunió començarà demà a les set.
- La reunió demà començarà a les set. (less neutral)
- A les set començarà la reunió demà. (possible, but less likely in normal conversation)
Starting with demà sounds especially natural when the speaker wants to foreground when it happens.
Because reunió is a countable noun, and Catalan usually uses an article with it when referring to a specific meeting.
- la reunió = the meeting
- una reunió = a meeting
So la reunió tells you both speaker and listener know which meeting is meant. Leaving the article out would normally sound ungrammatical in this sentence.