I hate Mondays!

Javascript - 47 45 43 42 34 32 29 bytes

8 bytes saved thanks to @Downgoat.

5 bytes saved thanks to @Dendrobium.

f=_=>(Date()[0]!="M"&&"f=")+f

Jolf, 19 13 9 bytes

?wfb+q~2q

Try it online.

Saved quite a few bytes thanks to Conor.

?wfb+q~2q

?wfb+q~2q
?          if
 wfb       the current weekday - 1 (which is 0 on Monday) is truthy
    +q~2   print the program's source code with 1000 appended to it
           else
        q  print the program's source code

CJam, 18 17 bytes

{"_~"et7=({\}|}_~

Test it here.

On Mondays, it prints:

_~{"_~"et7=({\}|}

Dennis found a 16-byte solution which will break in the year 10,000 though:

{"_~"et_7=1=<}_~

On Mondays, this prints

{"_~"et_7=1=<}_~2016

... or whatever the current year is.