Learn Russian: Сегодня четырнадцатое мая, это мой любимый месяц. - Today is the fourteenth of May, it is my favorite month.

When saying what the date is today, Russian uses a neuter ordinal number (like "fourteenth" instead of "fourteen"). It takes the neuter -ое ending because it is agreeing with the neuter noun число (date/number), which is understood but usually left out of the sentence.

Why is the month мая instead of its dictionary form май?

The month takes the genitive case, which is often the Russian equivalent of the English word "of". You are literally saying "the fourteenth [day] of May".

The word for month, месяц, is masculine. Why use the neuter word это to say "it is" instead of он?

In this sentence, это is used as a generic pointer meaning "this is" or "it is" to state a fact. When used as a standalone pointer like this, это doesn't change gender. However, the describing words that belong to the month, мой любимый (my favorite), do take masculine endings to match the masculine noun месяц.

What is the difference between четырнадцать and четырнадцатое?

Четырнадцать is the regular, cardinal number "fourteen". Четырнадцатое is the ordinal number "fourteenth". In Russian, ordinal numbers behave just like adjectives, meaning they change their endings to match the gender, case, and number of the noun they describe.