Mainly your environment. In spoken French only, people will use "on est" in place of "nous sommes" in an informal context. How frequently people use "on est" in place of "nous sommes" really depends on the acceptable level of informality in a given environment. The true use of "on est" is the impersonal singular person, like "one is" or "someone is".
The verb "être" is conjugated as follows: Je suis Tu es Il/elle/on est Nous sommes Vous êtes Ils/elles sont
Aujourd'hui, c'est [insert current day of the week].
To ask for today's date in French, you would say: "Quelle est la date d'aujourd'hui ?"
To ask "What day is today?" in French, you would say "Quel jour sommes-nous aujourd'hui ?"
In French, "is" is translated as "est." This is the third person singular form of the verb "être," which means "to be" in English. It is important to note that French verbs change depending on the subject pronoun, so "est" is used with il, elle, or on (he, she, or one) as the subject.
The phrase "Nous sommes est fort en chimie" is not grammatically correct in French. It seems to be a mix between "Nous sommes forts en chimie" (We are good at chemistry) and "Il est fort en chimie" (He is good at chemistry).
Nous sommes le combien aujourd'hui? Quelle est la date d'aujourd'hui? Nous sommes le quoi aujourd'hui?
"Love is why we are here"
quelle est la date d'aujourd'hui, le combien sommes-nous aujourd'hui, quel jour sommes-nous aujourd'hui are common ways of asking what is the date today? in French.
on est allées (better: "nous sommes allées")
The verb "être" is conjugated as follows: Je suis Tu es Il/elle/on est Nous sommes Vous êtes Ils/elles sont
"Est" is bassically what you would use in the context tu es, nous sommes, vous êtes, ils/elles sont
to be = être i am = je suis you are = tu es he/she is = il/elle est we are = nous sommes you are = vous êtes they are = ils/elles sont
to be is the verb "être" in French je suis tu es il, elle est nous sommes vous êtes ils, elles sont
Aujourd'hui, c'est [insert current day of the week].
J suis Tu es Il/elle est Nous sommes Vous etes Ils/elles sont
no that means nothing. if you want to say "what day is it ?" you should say : "quel jour sommes-nous ?" or "quel jour est-on ?"