Questions & Answers about Я думаю, сегодня семнадцатое октября.
Why does семнадцатое end in -ое?
To give the date, Russian uses ordinal numbers (first, second, third), which act like adjectives. Here it takes the neuter ending -ое because it implicitly agrees with the neuter word число (number or date), which is left out. You are literally saying 'the seventeenth [date]'.
Why is the month written as октября and not октябрь?
When saying a date, the month is put into the genitive case to express 'of' October. Октябрь is a masculine noun ending in a soft sign (ь). To form the genitive, you drop the ь and add -я.
Why is there a comma after думаю?
Even though this sentence drops the word что (that), it still connects two separate parts of the sentence: 'I think' and 'today is'. Russian punctuation rules strictly require a comma between them, unlike in English where 'I think today is' needs no comma.
Why is there no word for 'is' before the date?
In the present tense, Russian almost always drops the verb 'to be' (am/is/are). You just put the words next to each other, so 'today is the seventeenth' is simply сегодня семнадцатое.