This will happen at a particular time in the future. It will rain tomorrow. It is going to rain tomorrow. Maria will eat fish next Friday. Maria is going to eat fish next Friday.
This tense refers to something which began and ended in the past. Quite simply, this occurred at one particular time in the past. It rained 2 days ago. She ate fish last Friday.
The Simple Present tense expresses situations or events which exist always, usually or routinely. They exist now, they have existed in the past, and will most likely exist in the future. It rains in Spain. Maria eats fish every Friday.