What to do if i was diagnosed with hiatal hernia and i'm supposed to fast for religious duties soon?

No problem: Enjoy your fast. There are no issues.

Nothing special: Most title hernias are small and not dangerous for any type of activity or diet, including fasting. Large hiatal hernias can be associated with severe symptoms however, and should get checked out. These are also known as paraesophageal hernias.

Should be okay ...: if your hernia is not causing you symptoms then I wouldn't worry about it. I am assuming you will abstain from food and/or water for a prescribed amount of time. This should not aggravate your hernia. If you have other symptoms related to the hernia, such as reflux, then this will probably continue through the fast.

Hiatal hernia: You don't have to do anything. You can still fast. If the hiatal hernia doesn't bother you, great. If it does, just take some over the counter Omeprazole.