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It should be fine tomorrow.

It must be fine tomorrow.

It should be a nice day tomorrow.

It must be a nice day tomorrow.

It should be a nice day.

It must be a nice day.

May I ask which of those sentences using should/must are correct? Thank you for your reply!
Comments 
All are grammatically correct.

They have different emphases - "should be" - will probably be; "must be" = sense of desparation "It must be a fine day tomorrow" >>"It had better be a fine day tomorrow for my wedding or it will be spoilt!"
Hi Feebs11

When you said the version using 'must be' gives a sense of desperation, did you mean something like 'wanting or needing something very much' as in desperate for a fine day?

Thanks Emotion: smile
PBF
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It should be [fine / nice / a nice day] tomorrow. = In my estimation, it will be [fine / nice / a nice day] tomorrow.
It must be [fine / ...] tomorrow. = The day will be ruined if it is not [fine / ...] tomorrow. [much less used, in my opinion; occurs only in special situations]

CJ