While the team at Niantic is all set to begin celebrating their first ever in-game event for Pokémon GO to celebrate Halloween, the game has also gotten a nice little update prior to event, which is due to start on October 26th. The update while small as it holds mostly just a handful of game adjustments and no major feature adds, was likely to help smooth over the gameplay and perhaps to lay the groundwork for the event content that will go live in a couple of days. With that said, the update should be hitting users fairly close to now as Niantic have already started pushing it to devices, although it is worth pointing out that it's entirely possible that not all users will see the update today.
As for what's been changed, players may find that Niantic has fixed some of the text in the game UI, although if you have never noticed any issues with the text before now then you likely won't notice the fixes either. Beyond that, it appears that there are now going to be some new Pokémon type icons that were added to the information screen. Niantic states that there will be icons for each Pokémon that will be listed, so you'll probably see a lot of these.
If you're one of the lucky people that have been able to get a hold of the Pokémon GO Plus wearable, then there is now a low-battery indicator that has been added so that you can tell when your devices needs to be charged. Presumably, the wearable will blink when your phone battery is close to dying. The last little tweak that was made is in reference to Professor Willow, and mentions that he has now discovered that eggs have patterns on them that are unique, based on how far of a distance the user had to travel to get them hatched. What this means is that players will likely see an egg with a completely different pattern from another if they had to walk a different distance to hatch it, so this is a small visual adjustment here.