Hi everyone! Here I put some useful calendars. Those who wish to add their useful calendars can write me in PM or leave a comment.

TopCoder

Simply select the category competitions, which are interesting to you personally, and add the resulting calendar in iCal format to your Google calendar, Yandex calendar or any program-organizer

Codeforces

Сodeforces events calendar

Google Codejam

Automatically generated, customizable to your taste, anywhere to import Google Codejam events calendar

In many links (calendars) in the lower right corner there is a button add the calendar to your Google calendar. Thereafter, all updates will be automatically displayed in your Google calendar
If you do not use Google Calendar, you can watch/monitor/import the calendar in iCal or XML format.
And if you are using, do not forget that you can put a sms reminder (free) or e-mail - so now forget about the round is simply unthinkable! And you can do (in preferences) once for all events on this calendar, rather than manually for each individual, highly recommend!