Is she trying to remember 'strings of letters'? Rather than 'which letters represent the sounds I hear in this word'? Remembering strings of letters is very hard.
I would suggest looking at each word/weekday and determine which parts are 'easy' i.e. have a straightforward PGC (a straightforward way in which the letters in the word and the sounds you say when you speak the word correspond). Then look at the 'tricky' parts where there are several ways to write that sound, and/or an unusual way to write that sound. That is then the only bit she needs to remember, and perhaps some mnemonics could help?
E.g. Monday
m, n, d ->straightforward.
ay -> she has probably learned that the /ai/ sound at the end of a word is usually/often spelled ay. In any case, all seven weekdays end on 'day' so that should not be too hard (and seeing as she can do two of the weekdays, she should be able to do that part in the other days too)
That leaves o for the /u/ sound which is a bit tricky. It is a fairly unusual spelling for this sound and most likely not been taught yet. So she has to remember that in Monday, the /u/ sound is spelled with an o and the rest is straightforward.
Mnemonics for this, depends what other words she knows how to spell. E.g. if she knows 'mother' (where the /u/ sound has the same spelling o) you could say something like 'on Monday, other mothers eat onions' (just made this up) giving her at the same time a way to remember a few words where the /u/ sound is spelled with an o.
However you say she has Monday down, but no clue for Sunday. Sunday ought to be easier, as each sound in the word is spelled with the letters she should have learned for those sounds. This really implies, to me, that she is not using letters to 'encode' the sounds in the words, but is merely trying to remember 'strings of letters'. Also you say she struggles to spell other 'easy' words.
This kind of raises the question:
- Has no-one shown her that she can use her phonics knowledge to 'encode' (spell) rather than just to 'decode' (read)? Can she not properly hear the sounds in the words/process those sounds into the phonics she has learned? Or does she just not understand that this is what she should be doing?
-> in which case the way to help her would be to move away from the 'strings of letters' and really focus on the sounds in the words and the letters that represent them. And if you find she struggles to 'hear' the sounds or the process them, then raise this with the teacher.
- Or is her phonics knowledge poor? If you ask her 'what letter makes the sound /u/ - could she reliably answer that? Does she know that /ai/ can be spelled ai, ay, a-e (and other ways)?
-> if her basic phonics knowledge is the problem, I would forget about learning spellings for now and really, really focus on patching up her phonics. Once she's got that, learning spellings will be so much easier.