If you think Monday's are tough, wait until next week!

President Reagan has a shopping list (his new budget) going to Congress next Monday that will keep federal union lobbyists busy--and make some civil servants uncivil--until mid-May when the budget starts taking final form on Capitol Hill.

Thanks to a variety of leaks--some calculated, some not--we know that the President will ask Congress to:

* Go along with a zero federal pay raise this October, even if government data show Uncle Sam slipping further behind the private sector in the salary department.

* Make major changes in the federal retirement system that would require employes not yet 55 with 30 years of service to work longer, and pay more into their retirement fund.

* Increase the retirement deduction from federal and postal paychecks from the present 7 percent of gross salary to 9 percent next year, and 11 percent in 1985.

The pay freeze would not stop automatic longevity step increases (worth about 3 percent) that employes get regularly every one, two or three years. But it would mean no October adjustment (workers got 4 percent last year).

Congress will probably block some of the president's federal personnel proposals--although many members favor some of them--but some almost certainly will get through.

Most unions will concentrate their efforts toward blocking mandatory Social Security coverage. Although the president has not proposed such coverage, GOP leaders in the Senate and House Democratic leaders have tentatively endorsed the concept. The legislation would have to go through the House Ways and Means Committee and Senate Finance Committee.

Unions are counting on the friendlier House Post Office-Civil Service Committee to block or delay any presidentially proposed pay freeze or pension system change--until the Social Security fight is either won or lost. Then they will concentrate on pay and retirement changes.