Learn Portuguese (Portugal): Fico no seminário desde o meio-dia até à noite. - I stay at the seminar from noon until night.

In Portuguese, time expressions like meio-dia (noon) usually take a definite article. So, instead of saying just 'desde meio-dia' (from noon), you say desde o meio-dia (literally: from the noon).

Why is it 'até à noite' with a grave accent, instead of just 'até a noite'?

In European Portuguese, the preposition até (until) is typically followed by the preposition a (to). Since noite is a feminine noun, it also needs the feminine article a (the). The preposition a and the article a merge into à.

Could I use 'de' and 'a' instead of 'desde' and 'até'?

Yes, you could say do meio-dia à noite. The pair de... a... is very common for standard 'from X to Y' times. The pair desde... até... used in this sentence just adds a bit more emphasis on the continuous duration of staying there.

Why does the sentence use 'Fico' instead of 'Estou'?

While estou means 'I am', the verb ficar often translates to 'to stay' or 'to remain'. Because you are talking about staying at the seminar for a specific block of time, Fico is the most natural verb to use here.