This post assumes that readers already have some basic knowledge of Thai.
The main argument in this post seems to hold true in Lao also. So, if you are a Lao-language learner, do read on.
This post is related to a larger issue in language learning: should a second language learner learn and use "classroom/formal" words or should he/she learn and use "real world" words?
In sum, I think that in spoken speech, you should use athit (อาทิตย์) not sapda (สัปดาห์) to say "week". This is because most Thais use the former not the latter. You will also connect with more people if you use athit. You should still learn what sapda means so that you read Thai texts and so that you can understand the people who actually do use it.
So, which one should you, as a Thai-language learner, use when conversing with a Thai person?
If you, for example, want to say "we went to Chiang Mai last week", which phrase would you use?
A) อาทิตย์ที่แล้ว เราไปเที่ยวเชียงใหม่มา
B) สัปดาห์ที่แล้ว เราไปเที่ยวเชียงใหม่มา
Well, you can use whichever you want, but if I were you, I would use athit not sapda. So, I would go with A. I would use athit simply because most Thais use it. To be part of the Thai-speaking community, it is a good idea to use whatever words that most Thais use. Using sapda might make you sound snobby and pretentious, and consequently you will set yourself apart from members from the Thai-speaking community.
Have you ever tried speaking like a small child when you are speaking to a child? For example, have you ever said "do you love your mommy?" or "does your tummy hurt?" to a child? In general, when you and your listener use the same words, or as close/similar words as possible, the two of you will likely to connect more; this way you are creating solidarity with the listener. This might be one of the reasons why a person code-switches to make his/her speech sound like that of his/her listeners. This way he/she is narrowing the social distance between himself/herself and his/her audience. Deliberately making one's speech different from that of the listener can cause the opposite effect. Particularly, when a person makes his/her speech different from that of the audience, he/she is making himself/herself different from the audience. Therefore, using athit in spoken speech is a good idea!
This does not mean that knowing what sapda means is useless; you should still know what it means because you might see this word in the written language, or you might actually hear somebody use it in speech. Not knowing what sapda means could make it hard for you to get by in Thai settings.
Now another questions arises: "but wait, doesn't athit mean 'Sunday'"? Well yes and no because the full word for "Sunday" is wan athit (วันอาทิตย์); you actually need the word for "day" (wan วัน) to disambiguate between "Sunday" and "week".
This preference seems to hold true for Lao as well. Lao also has athit (ອາທິດ) and sapda (ສັບດາ sometimes pronounced sappada ສັບປະດາ, and less commonly sattawaan ສັດຕະວານ, sattaha ສັດຕາຫະ)
In Shan, my dictionary tells me that the words for "week" are wong (ဝူင်ႈ ว่ง) and pat (ပၢတ်ႈ ป้าด), but a Shan person has informed me that the first one is more common.