Word or short phrase meaning "Happening on the same day as"
There is wide usage of the hyphenated variation "same-day," though it is largely associated with commercial services, as in "Same-Day Delivery." Most languages are rife with loanwords, though, and borrowing is almost always cheaper than building.
simultidian - a portmanteau of simultaneous and quotidian. (You didn't use the [real-word] tag.)