Nowruz is the Persian New Year. The number 7 and the letter S are related to the seven Ameshasepantas as mentioned in the Zend-Avesta. 

 seven things beginning with the letter sin (س)) are:

Sabze (Persian: سبزه) – wheat, barley, mung bean, or lentil sprouts grown in a dish.
Samanu (Persian: سمنو) – sweet pudding made from wheat germ
Persian olive (Persian: سنجد, romanized: senjed)
Vinegar (Persian: سرکه, romanized: serke)
Apple (Persian: سیب, romanized: sib)
Garlic (Persian: سیر, romanized: sir)
Sumac (Persian: سماق, romanized: somāgh)

There are other elements that can be added to the Haft-sin such as colored eggs, the Holy book and etc.