IMHO, here is no substitute for seeing the Botafumeiro "fly" after you have completed an arduous Camino, at least once. This is particularly the case if you are an observant Christian, or anyone who appreciates the history and traditions behind the use of this oversized censer.
However, and NOT trying to be snarky, you can easily see the Botafumeiro fly on You Tube. There are probably hundreds of videos shot by pilgrims and tourists available there. The only difference is that you are not THERE. Smell is optional too. The incense does have an odor...
This said, and counting myself in the "cranky crowd" as well, I will impart one piece of advice. As has been said already, the Cathedral sponsors using the Botafumeiro on Catholic holy days and other significant days as determined by the Archbishop. Again, as stated above, these dates ARE published in advance. You COULD plan your Camino around these dates.
Alternatively, I have found over five-years that determining if some large group or organization will be attending Mass at the Cathedral for some purpose or another is a very good indicator that the Botafumeiro will be used. For example, each June, the Spanish military holds a commemorative Mass, or several. The Botafumeiro is used.
Throughout "the season" (Easter to October) one pilgrim or tourist group, or another entity will pay to have the Botafumeiro used at a Mass the group attends. For example, a large group of pilgrims from a parish, civic organization, club, or pilgrim society will sponsor the Botafumeiro. If you can watch the local newspaper for upcoming events like this, it would also be a good indicator that the Botafumeiro. Conversely, a tourist company, having brought several hundred tourists into Santiago, might easily pay the cost to have the Botafumeiro used.
Just as an FYI, the six fellows who pull the ropes also clean and prepare the Botafumeiro for each use. They are lay people (non-religious) who must be paid. I was informed a couple of years ago, that the overall cost for all aspects of the Botafumeiro ceremony is about €400. This explains why the Cathedral is not able to afford to fly it everyday.
In previous years, the local hoteliers & restaurant association funded the Friday 19:30 Mass use of the Botafumeiro. Their reason was simple, people would come to Mass to see the Botafumeiro. Then these folks would eat in the hundreds of fine restaurants and stay in their hotels. However, and as indicated by one posted above, this practice has apparently been discontinued. So, we are back to chaos theory and chance...
If anyone has the straight story on the Friday evening use issue, please contribute it to the dialog. I will return to work as a volunteer for a month at the Pilgrim Office, in three weeks. When I arrive, I will certainly learn the current state of affairs. But, knowing sooner, and in the forum, would help others.
I hope this helps.