This morning (September 1st) I completed my 7th field research task and I was expecting to get an Entei. Instead I got a Raikou. I thought that at the beginning of each month the pokemons switched to the new one. How come this happened?
So, you can complete the task, but just don't claim the reward for that task before 1pm PST / 9pm UK time of the first of the month, as doing so will activate the arrival of the Special Research reward at the top of the screen.
If it's before then, it'll be the reward for the month before (so in April, this was Moltres). Wait until after, it'll be the next month's (so in May, this was Zapdos).
To clarify further, this is when you complete the task, not open it. Even if you then open it after the change, it'll be the prior month's Pokémon. To be safe, wait until it's the new month if you want to get the latest creature.
So effectively, you completed your 7th task too soon. If you wanted an Entei, you should have waited until after the rollover moment.