Generally, when travelling on days of major celebrations, it is hard to estimate the level of celebrations that may take place in transit hubs such as bus stations or airports. Halloween, while it is celebrated in much of Uruguay and Argentina similarly to the United States, is still not considered a major holiday, which reduces the possibility of any forms of major disruptions due to Halloween. Of course, this also depends on whether there are individuals present in your bus or in the bus stations you will be celebrating the "Halloween" Spirit.
I have travelled previously on Halloween and Christmas in the United States and United Kingdom by air, on various New Year's Eves/Days in a bunch of different countries also by air, and on Diwali in India by air and train and have not noticed any celebrations that are officially sanctioned, other than decorations (and carol singing on Christmas). Only on one occasion was I accosted by a drunk tourist in an airport lounge in Georgia on Orthodox New Year (14 January) and, even then, I was asked whether I wanted to try a local alcoholic beverage.
In most cases, since the number of people actually travelling on the day of the major holiday is less (as most people decide to travel in the period immediately before or after the holiday), I have noticed that airports and train stations are surprisingly quiet and processes such as security and immigration move faster.
In short, it is extremely hard to predict the extent of travel disruptions during holidays, especially those that are minor holidays in a region. The chances of these disruptions can greatly vary based on whether the individuals immediately around you are celebrating the holiday or not.