Once a month, a region and mission are selected from the "alien strategy" table. You can see the relative weights for these in /path/to/openxcom/standard/xcom1/regions.rul and which races conduct these missions in alienMissions.rul. As the game progresses, mission+region combinations are removed from the table when they are chosen, and when the table is completely empty, it regenerates from the ruleset file settings. You'll notice that this means you don't really have control over which missions happen over time, only which races conduct this mission by month.
However, you can use the scripts to determine which missions are run in a given month; the missionWeights parameter allows you to create a weighted list of missions by month when this script is run, and if you have "useTable: true" on the script, the mission chosen by the script will be run instead any chosen by the alien strategy table.