Hello Karim Lazaar Robbert M.Junaid, here's a simple solution(tested and approved 😉) with explanation

To filter a list for a particular day from Firestore documents, you can use the Less Than or Equal to and Greater Than or Equal To options to create a date range query. Here's how you can achieve this:

Create a DateTime Range

- In the FlutterFlow UI, create a variable to store the selected date.

- Use the calendarSelectedDay variable to calculate the start and end of the day.

The Firestore Query

Add a Firestore Collection widget and configure the query.
Select the \"date\" field to use for filtering.
For the relation, select Greater Than or Equal To and set the value to the calendarSelectedDay variable with \"Start\" as the available option.
Add the next filter. Select the same field again.
For the relation, select Less Than or Equal To and set the value to the calendarSelectedDay variable with \"End\" as the available option.

Ensure the Condition that Ties the Two Filters is AND
- Illustrations
Filter 1
Filter 2
The condition

Filtering data by date in calendar