I'll start with tradition. It can be both countable and uncountable. According to Macmillan Dictionary, it is uncountable when it has the following meaning:
very old customs, beliefs, or stories, considered together
Here are a few examples from different dictionaries:
According to family tradition, Mr Thomas was a teacher.
By tradition, it’s the bride’s parents who pay for the wedding.
The move represents a break with tradition.
As for mood, the Cambridge dictionary says it can be considered both countable and uncountable, too. I agree that the indefinite article is usually expected before a singular indefinite mood. And according to other dictionaries I have used (Collins and Longman), mood is always countable. Anyway, if we say "in a festive mood", it's certainly OK. Let's attribute the lack of a before mood in your example to that rare point of view the Cambridge dictionary has noted (unless someone can give a better explanation).