Time

Days/ Weeks/ Months/ Years

Kanji	English	Context
先	previous/ before	Used in 先週 (last week)
今	now	Used in 今週 (this week) and 今日 (today)
来	next	Used in 来週 (next week)
昨	previous	Used in 昨日 (yesterday) The reason why yesterday uses 昨 instead of 先 like weeks do, is because yesterday is a single day while a week is a period of time.
明	bright	Used in 明日 (tomorrow) The reason why tomorrow uses 明 instead of 来 like weeks do, is because tomorrow is a single day while a week is a period of time. It's also interesting to note that the kanji 明 (bright) is used because Japanese like to image that tomorrow will be a good day.
間	interval	Used in 三週間前 (3 weeks ago) Used to describe a period of time. Weeks, months and years use it 三週間前 (3 weeks ago), days however not 三日前 (3 days ago). And example of where days use it would be 三日間の旅行 (a three-day trip).
数(not a kanji)	several	Used in 数日前 (several days ago)
前	ago/ before	Used in 数日前 (several days ago)

(English - Kanji - Hiragana)

Days

Weeks

day	日	ひ
yesterday	昨日	きのう
today	今日	きょう
tomorrow	明日	あした
3 days ago	三日前	みっかまえ
several / some / a couple of days ago	数日前	すうじつまえ

week	週	しゅう
last week	先週	せんしゅう
this week	今週	こんしゅう
next week	来週	らいしゅう
3 weeks ago	三週間前	さんしゅうかんまえ
several / some / a couple of weeks ago	数週間前	すうしゅうかんまえ

Months

Years

month	月	つき
last month	先月	せんげつ
this month	今月	こんげつ
next month	来月	らいげつ
3 months ago	三ヶ月前	さんかげつまえ
several / some / a couple of months ago	数ヶ月前	すうかげつまえ

year	年	とし
last year	昨年	さくねん
this year	今年	ことし
next year	来年	らいねん
3 years ago	三年前	さんねんまえ
several / some / a couple of years ago	数年前	すうねんまえ