Learn Russian: Вчера было двадцатое марта, было тепло. - Yesterday was the twentieth of March, it was warm.

When saying the date in Russian, the ordinal number is actually describing the hidden word число (date/number). Because число is a neuter noun, the number takes the neuter ending -ое to become двадцатое.

Why does the month end in -а (марта)?

To say 'the twentieth OF March', Russian uses the Genitive case. Март is a masculine noun, so we add -а to the end to make it марта.

Why is the verb было used twice, and why is it in the neuter form both times?

The first было agrees with the neuter date двадцатое (twentieth). The second было is part of the phrase было тепло (it was warm). When talking about weather generally without a specific subject like 'it', Russian uses the neuter past tense.

Why do we use тепло instead of the adjective тёплый?

When describing general states or weather conditions like 'it is warm', Russian uses adverbs ending in -о rather than adjectives. Because there is no specific noun being described as warm in this part of the sentence, we use the adverb тепло.