Questions & Answers about Сегодня первое число.
Because первое agrees with the noun число, which is neuter (оно). In Russian, ordinal numbers behave like adjectives and must match the noun in gender, number, and case:
- первое число (neuter)
- первый день (masculine)
- первая неделя (feminine)
Yes, literally число = number, but in everyday Russian число also means the calendar date (day of the month).
So Сегодня первое число means “Today is the first (of the month).”
In the present tense Russian usually omits the verb to be (есть).
So Сегодня первое число is essentially “Today (is) the first date.”
You can say Сегодня есть первое число, but it sounds unnatural in normal conversation.