Hi
1- À quelle heure....
3- Qui célèbre ce jour férié ?
for celebration, if it's a day off, you can say "jour férié" as you wrote "holiday"...
celebration = you can either use "fête" or "célébration". "Fête" is mor comonly used than "célébration". Depends on the context.
Fête des mères
Fête de Noël
......
célébration looks more religious..
hope it helps :)