A very cold day). The ending agrees for gender and number: "Quadri costosissimi. Frutta dolcissima. Amiche carissime" ( Very expensive paintings. Very sweet fruit. Very dear friends f. pl.)
I superlativi
It. grammar distinguishes between "absolute" and "relative" superlatives.
Il superlativo assoluto (the "very + adj./adv." superlative)
The most typical form uses the ending -issimo. Examples: "Un giorno freddissimo" ( A very cold day). The ending agrees for gender and number: "Quadri costosissimi. Frutta dolcissima. Amiche carissime" ( Very expensive paintings. Very sweet fruit. Very dear friends f. pl.)
The adverb "molto" is sometimes used: "Canta benissimo Canta molto bene ( He/she sings very well).
Occasionally, the adj. or adv. is reduplicated: "Si sedettero vicini vicini per ripararsi dal freddo" ( They sat very close to protect themselves from cold)
Il superlativo relativo (the "-est / most" superlative)
It resembles the "comparativo di maggioranza" 
but it is not a comparison between two elements: it compares one element with all the others in the same group. Examples:
1) comparative: "Eddo č pių grasso di Giampiero" ( Eddo is fatter than Giampiero)
2) superlative: "Eddo č il pių grasso dei miei colleghi" ( Eddo is the fattest of my colleagues)
Notice the presence of the definite article before the superlative: only this makes it different from the comparative.
Notice also the use of "di" (by itself or in one of the "articled" forms "del dello della dei degli" etc.) after the superlative. If there is a whole clause:
"Tiziano č la persona pių grassa che io conosca" ( Tiziano is the fattest person I know), then we have "che" instead of "di".
For the irregular forms of comparatives and superlatives see