先週/今週の任意の曜日の日付を取得する (e.g.今週の月曜日、先週の月曜日を取得する)

今週の月曜日を取得する。

this_day = Date.today
this_monday = this_day - (this_day.wday - 1) # 今週の月曜日
last_monday = (this_day - (this_day.wday - 1)) - 7 # 先週の月曜日

--実行結果
[1] pry(main)> this_day = Date.today
=> Wed, 05 Dec 2012
[2] pry(main)> this_monday = this_day - (this_day.wday - 1)
=> Mon, 03 Dec 2012
[24] pry(main)> last_monday = (this_day - (this_day.wday - 1)) - 7
=> Mon, 26 Nov 2012

ポイントは、wdayメソッドで日付を曜日分逆算すること。

Time#wdayメソッドの戻り値は、曜日を数値にした値です。
0(日曜日)〜6(土曜日)の値を返します。

[1] pry(main)> Time.now.strftime("%A")
=> "Wednesday"
[2] pry(main)> Time.now.wday
=> 3

日曜日 => 0
月曜日 => 1
火曜日 => 2
水曜日 => 3
木曜日 => 4
金曜日 => 5
土曜日 => 6

応用編

今週の任意の曜日の取得

this_sunday = this_day - (this_day.wday) # 今週の日曜日
this_monday = this_day - (this_day.wday - 1) # 今週の月曜日
this_tuesday = this_day - (this_day.wday - 2) # 今週の火曜日
this_wednesday = this_day - (this_day.wday - 3) # 今週の水曜日 ← 現在
this_thursday = this_day - (this_day.wday - 4) # 今週の木曜日 ↓ 未来 (未来日でも、”-"して取得する)
this_friday = this_day - (this_day.wday - 5) # 今週の金曜日
this_saturday = this_day - (this_day.wday - 6) # 今週の土曜日

--実行結果
[1] pry(main)> this_day = Date.today
=> Wed, 05 Dec 2012
[2] pry(main)> this_sunday = this_day - (this_day.wday)
=> Sun, 02 Dec 2012
[3] pry(main)> this_monday = this_day - (this_day.wday - 1) 
=> Mon, 03 Dec 2012
[4] pry(main)> this_tuesday = this_day - (this_day.wday - 2) 
=> Tue, 04 Dec 2012
[5] pry(main)> this_wednesday = this_day - (this_day.wday - 3) 
=> Wed, 05 Dec 2012
[6] pry(main)> this_thursday = this_day - (this_day.wday - 4)
=> Thu, 06 Dec 2012
[7] pry(main)> this_friday = this_day - (this_day.wday - 5)
=> Fri, 07 Dec 2012
[8] pry(main)> this_saturday = this_day - (this_day.wday - 6)
=> Sat, 08 Dec 2012

先週の任意の曜日の取得
更にそこから - 7 すると先週の日付が求められる。

last_sunday = this_sunday - 7
last_monday = this_monday - 7
last_tuesday = this_tuesday - 7
last_wednesday = this_wednesday - 7
last_thursday = this_thursday - 7
last_friday = this_friday - 7
last_this_saturday = this_saturday - 7

--実行結果
[9] pry(main)> last_sunday = this_sunday - 7
=> Sun, 25 Nov 2012
[10] pry(main)> last_monday = this_monday - 7
=> Mon, 26 Nov 2012
[11] pry(main)> last_tuesday = this_tuesday - 7
=> Tue, 27 Nov 2012
[12] pry(main)> last_wednesday = this_wednesday - 7
=> Wed, 28 Nov 2012
[13] pry(main)> last_thursday = this_thursday - 7
=> Thu, 29 Nov 2012
[14] pry(main)> last_friday = this_friday - 7
=> Fri, 30 Nov 2012
[15] pry(main)> last_this_saturday = this_saturday - 7
=> Sat, 01 Dec 2012
[16] pry(main)>