Learn Russian: Весенний день длинный.

In Russian, the present tense of the verb "to be" (is, am, are) is normally left out. You simply put the subject (день) and the adjective describing it (длинный) side by side.

Весенний and длинный are both masculine adjectives. Why do their endings look different?

Russian adjectives have "hard" and "soft" groups. Длинный is a standard hard adjective, so it takes the normal masculine ending -ый. Весенний belongs to the soft group, so its ending changes to the soft version, -ий.

In English, "spring" is a noun. Why does it have an adjective ending here?

English often stacks nouns together, like "spring day" or "apple juice". Russian almost never does this. Instead, the first noun is changed into an adjective. So the noun весна (spring) becomes the adjective весенний (relating to spring).

Since день ends in a soft sign (-ь), how can I tell if it is masculine or feminine?

Nouns ending in a soft sign can be either masculine or feminine, so you usually just have to memorize their gender. However, this sentence gives you a huge clue: both adjectives describing it (весенний and длинный) have masculine endings!