This is the 1252nd most frequent German word.
wissen
"Wissen" means "to know" in English.
Here 'wissen' is used in its conjugated form 'weiß' to indicate knowledge of a particular fact, which is explained in the clause 'dass morgen ein Feiertag ist.'
Ich weiß, dass morgen ein Feiertag ist.
I know that tomorrow is a holiday.
In this sentence, 'wissen' is conjugated to 'weißt' in the second person singular, used to inquire about someone's knowledge of a subject, specifically the location of the keys.
Weißt du, wo die Schlüssel sind?
Do you know where the keys are?
The verb 'wissen' in its conjugated form 'wissen' for the first person plural expresses lack of knowledge about a situation stated in the clause 'ob sie kommen wird.'