Why stomata open during the day and closes at night.

Stomata open during the day and close at night because:

During the day, plants need sunlight for photosynthesis. When stomata open, they allow carbon dioxide (CO₂) to enter the leaf, which is needed for making food.

At night, there is no sunlight, so photosynthesis cannot happen. To save water and prevent water loss through evaporation, the stomata close.

In simple words:
In day time stomata open for carbon dioxide and photosynthesis.
In night stomata close to save water.