2/ Firstly, understand that this is one of the riskiest ways to trade. The first day is the point where cards have their lowest supply and most price volatility. The trend of the vast majority of cards is to drop in price while they are in packs, so you're fighting against that.
3/ The most coins I have ever lost while trading has been from trading cards on their first day. I do not recommend doing this unless you have a good amount of experience trading.
4/ IF a promo has a long lead-up (ie days of loading screens) where people can save up packs, AND IF the promo has a lot of hype, AND IF there are lightning rounds, THEN new cards can often hit their cheapest point of the first day sometime during the 2nd or 3rd lightning round
5/ This is because the amount of supply coming onto the market at once from saved up packs, lightning rounds, and relists can overwhelm demand for a short period of time. People see prices continue to drop and are scared to buy.
6/ Eventually supply will slow down as pack openings slow. Prices will stop dropping and will reach their bottom typically during the 2nd or 3rd lightning round. Once prices start rising they will rise very quickly as buyers see the small rise and now have confidence to buy.
7/ For new cards with a lot of hype, typically prices keep rising all night until Europe goes to sleep, then prices will typically drop overnight and rise the next morning when Europe starts playing WL. This trend is most prominent with new attackers and some midfielders
8/ If you time buying right, you can make a lot of profit buying at the low during lightning rounds and selling a few hours later. You can also make profit buying at night while Europe is asleep and selling the next morning
9/ EA have been running a lot of 2 week promos lately. This trend often works the first week, but NOT the 2nd week. The 2nd week does not have saved up packs and is less hype, so fewer packs are opened. Prices usually do not rebound as much once lightning rounds end.
10/ Once in a while, a new promo team will have a card that is very undervalued initially. This is usually a less known player from a less popular club or league. One example from last year - FFS Dominguez. His ingame stats were better than the face stats suggest.
11/ He was easily buyable for 300k in the first 20 minutes, but rose to over 400k+ within an hour. You can make profit by quickly identifying cards like this, but again it's very risky as you can lose a lot if you're wrong.
12/ Cards are often released with a price range that is too low and they go extinct. There are a lot of nuances to trading with extinct cards that deserve its own thread, but be on the lookout for cards with price ranges that are too low. Again, this is risky as well.