Roodmas is actually a Catholic holiday that's celebrated on September 14th. The Pagan holiday that's celebrated on April 30th is Walpurgisnacht which precedes Beltane on May 1st. And Lammas is celebrated on August 1st, not July 31st.
Mulder pronounces the the name of Gaelic holiday of Samhain as "sam-hayne," like it's spelled in the Roman alphabet. The correct pronunciation is "sah-win."