Banner

The gold banner resets hourly at XX:00 (in most timezones - India would be every XX:30). 
To buy one unit you need 500 gold. To buy 10 at a time you need 5000 gold. To skip the spinning animation from banner, click the screen again. 
 
=== The pity system ===
You can get legendary, mythical and godly from pity. Godly pity will give you a random godly, the same happens with legendary and mythical units. I would recommend saving your gold for a banner that has what you want instead of 'increasing' pity as you will not get what you want from doing that. 
There is no pity for divine, ultimate, rare or common units. 
Pity resets when you spin the unit from banner. Pity resets when you reach pity. 
To get pity for:
* Legendary - you will need 25,000 gold to reach legendary pity.
* Mythical - you will need 100,000 gold to reach mythical pity.
* Godly - you will need 500,000 gold to reach godly pity.
Pity is not proportional to the new rarity chances from the release of Update 24. It is unknown if they will be reduced to match the chances of each unit.