They are essentially the same in meaning. Every comes from a contraction of "ever" and "each". Ever + each = every. Chaucer spelt it as "everich". The "ever" simply intensifies the sense of being part of a larger whole. But in modern English only "each" can be used with "of them" ("each of them" not "every of them".), and only each can be used on its own as a noun ("Each came prepared" not "every came prepared") Also, "every" is never used when there are only two parts of a larger whole. If there are only two cats, you can only use "each cat", not "every cat" If there are more than than two, then you can use "each" or "every". Since "years" will generally imply a larger number (the years one experiences) for most purposes "each year" OR "every year" will be equally acceptable.

Some people wrongly use them interchangeably. There is a difference. However, this is a technical point of view, and the mainstream users probably wrongly use the words in the interchangeable manner. So, please exercise caution when using the proper interpretation of this because if the source wrote it wrongly, you should interpret it wrongly too (because double negatives make a positive). The example below demonstrates the proper interpretation. --- Assume Tom and Harry leased a party club for 10 years. Tom is calculating the number of parties required to break even, because they need to cover the operating expenses and rental fees. Tom: We need 100 parties to cover EVERY year. Harry: You mean 10 parties per year for 10 years. The total would be 100 parties. Tom: We need 100 parties to cover EACH year. Harry: You mean 100 parties per year for 10 years. The total would be 1,000 parties.

Each is a way of seeing the members of a group as individuals,while every is a way of seeing a group as a series of members.