Nettla, like many other booking providers, uses a payment mechanism knows as "auth-capture". This means that payments are first ”reserved” to be ”charged later”.
If you see a charge on an unexpected date, for example 2-3 days before your session takes place, it means that you have booked something that takes place more than 6 days away from your initial booking date.
Nettla is able to reserve an amount on your bank account for 6 days, and after that have to charge your bank account. This does not affect your potential refund (if you cancel) as you will still receive that no matter if your payment is in a reserved or charged state.