I've finished Day One. Here are my top tips!
First off you absolutely need good walking shoes, there's going to be a lot of walking involved but it's worth it!
The event is from 10am-6pm which is incredibly long, you're not going to be playing for 8 hours straight so make sure you plan out bathroom breaks, food breaks and also places of rest! Look after your wellbeing
If you are playing a lot of it, have at least 8 incences, you'll need one for every hour.
You're going to be catching A LOT of pokemon so sort your storage out to have at least 100 spaces free or be lazy and spend real money to buy some
When your box gets full, search 4* and delete everything that isn't 4 that you do not want.
I'd advise using the quick catch glitch as well if you want to speed things up. You should go into a densley populated area with a lot of pokestops to refill your pokeballs on and about a 15 minute look so spawns can refill
Incense spawn one pokemon per minute I believe when walking in this event, click the incense pokemon before it changes to something else, special eyes on pokemon such as corsola which is not available easily in europe and can be shiny
Make sure you are stocked up on pokeballs, don't click on pokemon spawns after 9.45am as they will all reset at 10am and you don't want to lose this
If possible play with a friend and take advantage of the bonus XL candy that is available
Stock water bottles, portable charges and portable chargers, you're going to need them! Also have a backup area to play incase it rains (such as a shopping centre)
Partner up with people in a party, each challenge you complete gives you an extra counter at the pikachu with a hat, partner up in putting lures down also.
I'd say lastly, use raids in person as a good opportunity to meet people and walk with them so you can tackle the raids together and get a good necrozma!