diff --git a/CHANGES.rst b/CHANGES.rst
index 18b77c2..f628918 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -667,9 +667,9 @@ Fixes:
 - created sphinx documentation and started documenting development and goals.
   [garbas]
 
-- hook out github repository to http://readthedocs.org service so sphinx
+- hook out github repository to https://readthedocs.org service so sphinx
   documentation is generated on each commit (for master). Documentation can be
-  visible on: http://readthedocs.org/docs/icalendar/en/latest/
+  visible on: https://icalendar.readthedocs.io/en/latest/
   [garbas]
 
 
diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 37e0bc6..22fa1f7 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -10,7 +10,7 @@ These are some contribution examples
 
 - Extending the documentation.
 
-- Sponsor a Sprint (http://plone.org/events/sprints/whatis).
+- Sponsor a Sprint (https://plone.org/events/sprints/whatis).
 
 
 For pull requests, keep this in mind
diff --git a/bootstrap.py b/bootstrap.py
index 4732735..6f5193e 100644
--- a/bootstrap.py
+++ b/bootstrap.py
@@ -111,7 +111,7 @@ cmd = [sys.executable, '-c',
 find_links = os.environ.get(
     'bootstrap-testing-find-links',
     options.find_links or
-    ('http://downloads.buildout.org/'
+    ('https://downloads.buildout.org/'
      if options.accept_buildout_test_releases else None)
     )
 if find_links:
diff --git a/src/icalendar/parser_tools.py b/src/icalendar/parser_tools.py
index 18559e6..a06af36 100644
--- a/src/icalendar/parser_tools.py
+++ b/src/icalendar/parser_tools.py
@@ -23,7 +23,7 @@ def data_encode(data, encoding=DEFAULT_ENCODING):
     """Encode all datastructures to the given encoding.
     Currently unicode strings, dicts and lists are supported.
     """
-    # http://stackoverflow.com/questions/1254454/fastest-way-to-convert-a-dicts-keys-values-from-unicode-to-str
+    # https://stackoverflow.com/questions/1254454/fastest-way-to-convert-a-dicts-keys-values-from-unicode-to-str
     if isinstance(data, compat.unicode_type):
         return data.encode(encoding)
     elif isinstance(data, dict):
diff --git a/src/icalendar/tests/test_icalendar.py b/src/icalendar/tests/test_icalendar.py
index 24e5968..c645ae5 100644
--- a/src/icalendar/tests/test_icalendar.py
+++ b/src/icalendar/tests/test_icalendar.py
@@ -59,7 +59,7 @@ class IcalendarTestCase (unittest.TestCase):
              '123456789 123456789 123456789 123456789 ')
         )
 
-        # http://tools.ietf.org/html/rfc5545#section-3.3.11
+        # https://tools.ietf.org/html/rfc5545#section-3.3.11
         # An intentional formatted text line break MUST only be included in
         # a "TEXT" property value by representing the line break with the
         # character sequence of BACKSLASH, followed by a LATIN SMALL LETTER
diff --git a/src/icalendar/tests/test_timezoned.py b/src/icalendar/tests/test_timezoned.py
index c80abd4..1e297cf 100644
--- a/src/icalendar/tests/test_timezoned.py
+++ b/src/icalendar/tests/test_timezoned.py
@@ -109,7 +109,7 @@ class TestTimezoned(unittest.TestCase):
         event.add('attendee', 'franz')
         event.add('attendee', 'sepp')
         event.add('contact', 'Max Mustermann, 1010 Wien')
-        event.add('url', 'http://plone.org')
+        event.add('url', 'https://plone.org')
         cal.add_component(event)
 
         test_out = b'|'.join(cal.to_ical().splitlines())
diff --git a/src/icalendar/windows_to_olson.py b/src/icalendar/windows_to_olson.py
index 0cc8ef9..52d305c 100644
--- a/src/icalendar/windows_to_olson.py
+++ b/src/icalendar/windows_to_olson.py
@@ -5,8 +5,8 @@ The data is taken from the unicode consortium [0], the proposal and rationale
 for this mapping is also available at the unicode consortium [1].
 
 
-[0] http://www.unicode.org/cldr/charts/29/supplemental/zone_tzid.html
-[1] http://cldr.unicode.org/development/development-process/design-proposals/extended-windows-olson-zid-mapping  # noqa
+[0] https://www.unicode.org/cldr/cldr-aux/charts/29/supplemental/zone_tzid.html
+[1] https://cldr.unicode.org/development/development-process/design-proposals/extended-windows-olson-zid-mapping  # noqa
 """
 
 WINDOWS_TO_OLSON = {