When starting to study Japanese Kanji, people would start with simple ones. Not only for a foreigner, this method also applies to Japanese children when they enter elementary school. This is because simple ones can help people accustom with the basic forms of Kanji characters. When you have learnt them by heart, you can easily go further study with more complicated Kanji and in most cases, you will find them are structured with simple ones.

On the Kanji learning journey, you might encounter some of them very frequently:
日　月　火　水　木　金　土
On the calendars, on the agendas, in your working schedule…yes, they are Japanese names of the days of the week.

日 means the sun, a day in Japanese. Here it stands for the Sun, Sunday.

月means the moon, a month in Japanese. Here it stands for the moon, Monday.

火 means fire, flame, light in Japanese, Here it comes from 火星※, Mars, Tuesday.
※星 means star

水 means water. Here it comes from水星, Mercury, Wednesday.

木means tree, wood, timber. Here it comes from木星, Jupiter, Thursday.

金means gold, money, metal. Here it comes from金星, Venus, Friday.

土 means soil, earth. Here it comes from 土星, Saturn, Saturday.

The days of the week as we can see, are named after the Sun, and other familiar planets from the solar system, whereas the planets were originally named after ancient Roman gods. Tuesday was named for the god of wars, Mars. Wednesday was named for the god of financial gain, commerce, Mercury. Thursday was named after the god of the sky and thunder, also the king of the gods, Jupiter. Friday was named after the god of love, beauty, the famous and beloved goddess, Venus. Saturn, the god of wealth and agriculture, represents Saturday.