I've read that what rookies rank up into is random, and I know that there are specific plot-related missions.
After I returned from a mission I saved the game. I then went through the process of the normal between-mission activities and then went to select my next mission. I was alerted of alien abductions in three countries
- China: Easy
- Brazil: Very Difficult
- Canada: Very Difficult
I decided I didn't really like my options all that much and hit reload. I went through and made the same post-mission decisions/assignments/purchases as I had before and again went to select my next mission. I was presented the same three countries and difficulty levels as my options.
I reloaded again. This time, I changed my post-mission actions. Even still, I was given the same three options for missions.
Clearly, the next missions offered up are not completely random. So what, if anything, affects those choices?
I'm not so much concerned with the difficulty levels like this question, but more the general mission parameters.
Best Answer
The mission types and dates are determined at the time of the council report for the prior month. You can verify this by making a save just before the report, and one just after. UFO interceptions/landings, terror + council missions etc. will always be the same if you load the game just after but will change if you load the save before.
Update: as phantom42 notes and I've observed before also, the exact mission type will change upon reload. While council mission would always occur on May 7th, whether it's an escort/extraction/bomb defusal mission may still vary upon reloading. Similarly, type of requests from countries will vary upon reload.