"Ramadan is the holiest month in the Muslim calendar. It is the month in which Muslims believe Prophet Mohammad started receiving revelations from God. The Islamic calendar, like the Jewish calendar, is a lunar calendar and the start of the month changes from one year to the next. This year, Ramadan is expected to begin tomorrow Monday, May 6, 2019 and may end on June 6."
There was no islamic calendar at the time of Prophet. Hazrat Umar started it in 638 CE (16 hijra)
How did people know about ramdaan, hajj and friday prayers then?
And what does Quran refers to Ramdan in chapter 2. Name of a month or condition of a season?