I cannot seem to find reliable information anywhere so I ask you guys:
1) When will the Frostmourne server reset?
2) Will all characters be transfered to the Icecrown server automatically when it resets?
3) Is there a way to transfer chars before a reset ?
1) around 15-April-2022 (since it began on 15-april-2021 and it lasts 1 year)
2) you will need to transfer them manually.
For example, this is what people need to do to transfer the character from the previous "Frostmourne Season 1" (active since 15-april-2020 till 14-april-2021) to "Icecrown": https://www.warmane.com/account > service
Select "Frostmourne - S1" as realm
3) No, you need to wait Realm reset (So, around 15-April)