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The sun is shining, it really is going to be a great day.

This sentence uses the phrase to indicate how good the weather is, implying it is going to be a wonderful day.

Let's go fishing, it's going to be a perfect day.

In this context, the speaker uses the phrase to indicate a perfect day for a specific activity, which is fishing in this case.

If the forecast is correct, tomorrow will be a terrific day.

In this example, the phrase serves to show the anticipated perfect weather according to the weather forecast.

By looking at the clouds, you can tell it's going to be a fabulous day.

The phrase is used to express that someone can predict the weather by observing the clouds.

Although it's windy, it's going to be a splendid day to fly kites.

Used to show a subjective perspective based on the speaker's interest (kite-flying in this case) that makes the day perfect regardless of the wind.

After so much rain, today is going to be a brilliant day.

It is used to express relief and hopefulness that the weather has improved and the day will be great after a period of heavy rainfall.

Given that we are in spring, it is going to be a splendid day.

Here, the phrase indicates that the good day is attributed to the spring season.

With this warm weather, it's going to be a wonderful day for a picnic.

The speaker is using the phrase to express how the warm weather makes the day perfect for outdoor activities like a picnic.

Despite the cold, it's going to be a fabulous day for skiing.

In this context, the phrase is used to convey the idea that even though it's cold, it's a perfect day for skiing due to personal preference.

Even without sun, it could be a wonderful day if you let it be.

Used to emphasize that a day's greatness is not necessarily dictated by the weather, but also a person's attitude.