Sea o será

[BobRitter]

SpanishDict example:

Ayer tuve un día malísimo. Espero que hoy sea mejor.
Yesterday I had a very bad day. I hope today will be better.

Espero que hoy sea mejor. I hope today IS better.
Espero que hoy será mejor. I hope today WILL BE better.

¿Por que usan "ser" en lugar de "estar"?

Espero que hoy esté mejor.
Espero que hoy estuviere mejor.

Yo sei que porque el verbo "espero que" tiene que usar el subjetivo.

Como siempre, gracias a todos.

[aleCcowaN]

The day "es" because it has certain features and properties as a whole or as a bottom line and not "está" because because it is or will be temporarily in a certain state.

Espero que hoy sea mejor :good:
Espero que hoy será mejor :?:(:bad:)

You cannot use indicative in the second sentence because Spanish indicative, unlike English, requires the clause which uses it to be real or true or to be held as real or true. Otherwise, like when you introduce that clause -the "hoped" part- with "espero" to describe your expectations (notice "espero" is indicative because you "do hope"), Spanish indicative is not accepted then it defaults to Spanish subjunctive (that is why there seems to be so many "different" cases of its use).

Present subjunctive covers both present and future here.

[babymetal]

After the verb esperar, you always need the subjunctive in a dependent clause.

[BobRitter]

Entiendo, gracias.