Spanish language Q&A Forum

The kwiziq page "Ser vs Estar in Spanish: Using ser in Spanish (not estar) to talk about time, days, dates and seasons" (Ser vs Estar in Spanish: Using ser in Spanish (not estar) to talk about time, days, dates and seasons) says

   Hoy es lunes.  =  Today is Monday.  ("Today" is singular.)

but

   Son las tres de la tarde.  =  It's three o'clock in the afternoon.  ("It" is plural.)

Why are these different?

De quién was an option for this question: 

Mi perro, ________ me acuerdo a menudo, ha muerto recientemente.

However when I selected it I was marked wrong.

However according to this page

Using preposition a and de + el que, la que, los que, las que = to/of/from/about which/who (relative pronouns)

Should I have been marked right because quién doesn't necessarily need to pair with a lo/la?

Can I say "Adivino que si" or "Adivino que no"? Thanks. 

de punta en blanco - wordreference is giving me a different definition. could this be reviewed? thank you!