The days of the week are essential as they crop up all the time. Just remember to say 'Dydd' in front of each one. Also, it sometimes help to think of something in English to latch on to if you can't remember words.

Sunday - Dydd Sul.
Think of the sun or Solar.

Monday - Dydd Llun.
Think of the moon or lunar.

Tuesday - Dydd Mawrth.
Think of the planet Mars.

Wednesday - Dydd Mercher.
Think of the planet Mercury.

Thursday - Dydd Iau.
This is linked to Jove or Jupiter, but think of touching something hot and the exclamation 'Iai!' for Thursday.

Friday - Dydd Gwener.
Again this is linked to Venus. Think of a beautiful woman called Gwen.

Saturday - Dydd Sadwrn.
Think of the planet Saturn.