I remember that learning the days of the week in Thai was a bit tricky back when I was getting started. In order to master their spelling, I wrote the day and date in my notepad and kept my expenses in Thai for a while. Once you’ve written it a few times over a few days or weeks, it should stick. Remember that connecting language with real life communication and experience is the secret to retention.
If you find yourself having trouble remembering any of them than try connecting the day with an activity. For example, “On Mondays, I go to work.”, “On Fridays, I like to go watch movies.”
Once you can say them, start trying to write them. Spelling is tricky, so you’ll need to do it repeatedly over a period of time and or use some mnemonics to help you remember the spelling. What I did was try to pronounce all the silent letters in my head. So for จันทร์ I’d say stuff like Jantra (or จัน-ทะ-ระ) to myself to help me remember the spelling.