Learn German: Das Museum feiert heute das hundertste Jubiläum. - The museum is celebrating its 100th anniversary today.

For numbers 20 and above, German adds the suffix -st to the base number. So hundert (100) becomes hundertst-. Because it acts like an adjective following the definite article das, it takes the standard adjective ending -e for a neuter noun, resulting in hundertste.

The English translation says "its 100th anniversary", but the German sentence uses das instead of a word for "its". Why is that?

In German, it is very common to use a definite article like das (the) instead of a possessive word like sein (its) when the context makes it obvious whose anniversary it is. Since the museum is celebrating, it is naturally its own anniversary.

Why does the time word heute come before the anniversary?

In German sentences, expressions of time like heute (today) generally come as close to the conjugated verb as possible, and usually appear before the direct object (das hundertste Jubiläum).

What kind of word is Jubiläum and how do I know its gender?

Jubiläum means anniversary or jubilee. Nouns ending in -um or -äum are typically borrowed from Latin and are almost always neuter, which is why it uses the article das.