While 置（お）きに may appear to be a 副詞（ふくし） (adverb), it is actually a combination of 置（お）き, a 接尾後（せつびご） (suffix) meaning ‘opening’, and the 格助詞（かくじょし） (case marking particle) に. 置（お）きに (often without kanji) is an expression used to show that something happens ‘every (A)’, or ‘at intervals of (A)’.
As
おきに is technically a suffix, it will be attached to the end of numbers or counters that it is highlighting as being an ‘opening’.
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この薬（くすり）は一日（いちにち）おきに飲（の）んでください。
Please drink this medicine every other day. (Every second day)
一（い）か月（げつ）おきに病院（びょういん）に来（く）るように先生（せんせい）に言（い）われました。
I was told by the doctor to come to the hospital every other month. (Every second month)
二日（ふつか）おきに部屋（へや）の掃除（そうじ）をしないと気（き）が落（お）ち着（つ）かない。
I can't relax if I don't clean my room every two days.
It is very common to get
ごとに, and
おきに mixed up. While
ごとに has the literal meaning of ’every’,
おきに has the literal meaning of ‘opening’. Due to this,
おきに will not always mean ‘every’. Let’s look at some examples of when these two expressions are the same, and when they are not.
Discrete variables (things that are usually measured upon completion) - Only
ごとに may be used to express ‘every’. For example, a day, a week, or a month may easily be divided, but they are usually counted as whole units. Due to this,
ごとに will mean ‘every week’, while
おきに would actually mean ‘every second week’ (because there is an opening of one week).
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一日（いちにち）おきに目薬（めぐすり）を差（さ）す。
I use my eye drops every other day.
私（わたし）は出（で）かけるおきに、忘（わす）れ物（もの）をする。
Every other time I go out, I forget something.
Continuous variables (things that are usually measured in fractions) - Either
ごとに or
おきに may be used. For example, seconds, minutes, and hours are usually measured by fractions, especially considering sports and racing. In this case
ごとに will mean ‘every second’, while
おきに will mean ‘every opening of one second’ (disregarding all of the milliseconds, this means every second).
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毎日（まいにち）2時間（じかん）ごとに体（からだ）を動（うご）かしている。
I move my body every 2 hours, everyday.
毎日（まいにち）2時間（じかん）おきに体（からだ）を動（うご）かしている。
I move my body every other 2 hours, everyday.
This is a topic that native speakers frequently get wrong themselves, so don’t worry if it is a bit tricky! If you are unsure about which one to use, think of 置（お）きに as ‘every other’ for most purposes.