Gwyn Llywelyn will return to read the news bulletins - as he did on the station's first morning in 1977.
Hywel Gwynfryn, Dei Thomas and Richard Rees will join Aled Hughes for the morning to share anecdotes, and there will be clips from the first programme Helo Bobol.
Celebrations continue all day with Bore Cothi, Tommo and Geraint Lloyd.
It will also mark the end of Radio Cymru's pop-up station Mwy, which was launched in the run-up to the 40th birthday.
Some of BBC Radio Cymru's first presenters will return to the station on Tuesday for its 40th anniversary.