Can this be related with 0017143 this that I found also?

As a workaround click on "Mon" to make Monday the first day of week.
At the moment there is no configuration option for it.

Seems to be a setting fdow=1, in the javascript to get this to work accrding to
http://www.dynarch.com/jscal/

If one fix a patch for this what is sugested solution?

A solution using the current implementation (JavaScript) would have to introduce a new account preference and a global default for this setting.
e.g. $g_default_start_day_of_the_week = 1;

The implementation can't take place in stable version 1.2.x. as it requires a database schema change (new column in table mantis_user_pref_table)

A very modern approach would use the latest HTML5 features and throw away the whole JavaScript stuff.
Let the browser do the dirty work and use input type date(time) [1].
But there are just a few browsers at the moment supporting this feature in a way that is comparable to the current implementation.

Submitting a patch is always a good idea, as it increases the chances of improvement eventually making it into MantisBT core. All contributions are welcome and greatly appreciated.

Patch submissions can be made in several ways. In the order of preference:

1. Send us a Pull Request on our Github repository [2]
2. Attach a GIT patch to the issue
3. Attach a Unified Diff, clearly specifying the patch's base release

Kindly avoid to upload entire modified PHP files.

Please make sure that your submissions adhere to our Coding Guidelines [3], if they don't your patch might be rejected.

[1] http://www.w3schools.com/html/html5_form_input_types.asp
[2] https://github.com/mantisbt/mantisbt
[3] http://www.mantisbt.org/wiki/doku.php/mantisbt:coding_guidelines