diff --git a/lib/crono/period.rb b/lib/crono/period.rb
index 13c39a3..65d27e5 100644
--- a/lib/crono/period.rb
+++ b/lib/crono/period.rb
@@ -1,19 +1,20 @@
 module Crono
   # Period describe frequency of performing a task
   class Period
-    def initialize(period, at: nil)
+    DAYS = [:monday, :tuesday, :wednesday, :thursday, :friday, :saturday,
+            :sunday]
+
+    def initialize(period, at: nil, on: nil)
       @period = period
       @at_hour, @at_min = parse_at(at) if at
+      @on = parse_on(on) if on
     end
 
     def next(since: nil)
-      if since.nil?
-        @next = Time.now.change(time_atts)
-        return @next if @next.future?
-        since = Time.now
-      end
-
-      @period.since(since).change(time_atts)
+      return initial_next unless since
+      @next = @period.since(since)
+      @next = @next.beginning_of_week.advance(days: @on) if @on
+      @next.change(time_atts)
     end
 
     def description
@@ -22,6 +23,28 @@ module Crono
       desc
     end
 
+    private
+
+    def initial_next
+      next_time = initial_day.change(time_atts)
+      return next_time if next_time.future?
+      @period.from_now.change(time_atts)
+    end
+
+    def initial_day
+      return Time.now unless @on
+      day = Time.now.beginning_of_week.advance(days: @on)
+      return day if day.future?
+      @period.from_now.beginning_of_week.advance(days: @on)
+    end
+
+    def parse_on(on)
+      day_number = DAYS.index(on)
+      fail "Wrong 'on' day" unless day_number
+      fail "period should be at least 1 week to use 'on'" if @period < 1.week
+      day_number
+    end
+
     def parse_at(at)
       case at
       when String
@@ -34,8 +57,6 @@ module Crono
       end
     end
 
-    private
-
     def time_atts
       { hour: @at_hour, min: @at_min }.compact
     end
diff --git a/spec/period_spec.rb b/spec/period_spec.rb
index f7ac2a2..b8a8543 100644
--- a/spec/period_spec.rb
+++ b/spec/period_spec.rb
@@ -15,6 +15,43 @@ describe Crono::Period do
   end
 
   describe '#next' do
+    context 'in weakly basis' do
+      it "should raise error if 'on' is wrong" do
+        expect { @period = Crono::Period.new(7.days, on: :bad_day) }
+          .to raise_error("Wrong 'on' day")
+      end
+
+      it 'should raise error when period is less than 1 week' do
+        expect { @period = Crono::Period.new(6.days, on: :monday) }
+          .to raise_error("period should be at least 1 week to use 'on'")
+      end
+
+      it "should return a 'on' day" do
+        @period = Crono::Period.new(1.week, on: :thursday, at: '15:30')
+        current_week = Time.now.beginning_of_week
+        last_run_time = current_week.advance(days: 1) # last run on the tuesday
+        next_run_at = Time.now.next_week.advance(days: 3)
+                      .change(hour: 15, min: 30)
+        expect(@period.next(since: last_run_time)).to be_eql(next_run_at)
+      end
+
+      it "should return a next week day 'on'" do
+        @period = Crono::Period.new(1.week, on: :thursday)
+        Timecop.freeze(Time.now.beginning_of_week.advance(days: 4)) do
+          expect(@period.next).to be_eql(Time.now.next_week.advance(days: 3))
+        end
+      end
+
+      it 'should return a current week day on the first run if not too late' do
+        @period = Crono::Period.new(7.days, on: :tuesday)
+        beginning_of_the_week = Time.now.beginning_of_week
+        tuesday = beginning_of_the_week.advance(days: 1)
+        Timecop.freeze(beginning_of_the_week) do
+          expect(@period.next).to be_eql(tuesday)
+        end
+      end
+    end
+
     context 'in daily basis' do
       it 'should return the time 2 days from now' do
         @period = Crono::Period.new(2.day)