Total War: Rome 2’s latest patch 13.1, entitled “Hotfixes and Festivals”, adds the usual bunch of new technical fixes and gameplay improvements, alongside new ”seasonal festival events” across all campaigns.
Most of the other changes relate to the recent Pirates & Raiders DLC, fixing problems with recruitment and unit stats. The patch should automatically download and install through Steam.
Regarding those mysterious seasonal events, here’s what a member of the Creative Assembly team had to say on the official forums; “These are events based on ancient festivities. They can randomly occur during the campaign and have individual effects, similar to other random events. All faction groups will have at least 2 new events, with the Greek factions getting 3.”
Here’s the full list of changes:
Technical and performance improvements
Campaign
A rare crash which occurred after declining to fight a battle as a reinforcing army while the camera was still moving on the Campaign map has been fixed. General battle improvements
Cavalry can now break away from infantry less easily in all battles.
Increased the cost of armoured elephants.
Improved terrain heights and building positions in Pydna Historical Battle.
Fix for raiding horsemen having wrong shield type.
Fix for Illyrian Raiders having wrong armour type.
Wind will now affect more particles during battles, e.g. smoke will be blown in the wind direction.
General campaign improvements
Added 9 new seasonal festival events across all Campaigns.
Some Getae objectives that were not completing now are.
Fix for Dacian Bowmen not being recruitable from upgraded buildings.
Fix for Hellenic wine subject choice giving the wrong effects.
Added new Balkan sword (melee weapon stats) for balancing and assigned to Thracian peltasts.
Bonus vs. large added to Rhomphaia and Falx.
Buffed Thracian Peltasts in melee.
Illyrian hoplites and noble hoplites now have Hoplite Wall ability.
Swapped ships for Illyrian Raiders, added Illyrian Raider ship to all the correct port levels.
Fix for technology missing a link to previous technology.
Fix for melee attack penalty in battles for Balkan group generals units.
Added Thracian Warriors to settlement garrison units.
Fix for units not being available from all appropriate building levels.
Added naked warriors to Nervii unit roster.
Usability Improvements
General
Updated the encyclopaedia to reflect the latest changes e.g. include all special units. 
