I learnt to ask weekday as, "Jin tian xing qi ji?" With an optional "shi" after jin tian. So I want to know if the shi in "Jin tian shi ji hao?" is also optional. Also, can I say, "Jin tian ji ri?" using ri for date?
Jin tian (shi) xing qi ji = 今天(是)星期幾 = Today (is) what day of the week
shi (是) is not optional, it is the verb in the sentence, the optional part is to omit the verb shi (是)
Same thing in "Jin tian (shi) ji hao" which means "今天(是)几号" shi (是) is not optional, but omitting it is.
We don't say "Jin tian ji ri (今天幾日/天)?" when asking what day of the week, we always say 星期幾