Google-sheets – Finding multiple “most common values” in a column

I wrote a custom function in Google Apps Script that takes all the criteria you want it to have (and more) in consideration.

Code

function myPayments(paymentPlan, dateRange) {
  var output = [];  
  for(var i=0, iLen=dateRange.length; i<iLen; i++) {
    var checkDate = new Date(dateRange[i]);
    var weekDay = checkDate.getDay()+1, sum=0;
    for(var j=0, jLen=paymentPlan.length; j<jLen; j++) {
      if(paymentPlan[j][1] == weekDay) {   
        if(paymentPlan[j][2] === "" && paymentPlan[j][3] === "") {
          sum += paymentPlan[j][0];
        } else if(paymentPlan[j][2] === "") {
          if(checkDate <= new Date(paymentPlan[j][3])) {
            sum += paymentPlan[j][0];
          }
        } else if(paymentPlan[j][3] === "") {
          if(checkDate >= new Date(paymentPlan[j][2])) {
            sum += paymentPlan[j][0];
          }
        } else if(paymentPlan[j][2] !== "" && paymentPlan[j][3] !== "") {
          if(checkDate >= new Date(paymentPlan[j][2]) && 
            checkDate <= new Date(paymentPlan[j][3])) {
            sum += paymentPlan[j][0];
          }
        } 
      }
    }
    if(sum == 0) {
      output.push([]);
    } else {
      output.push([sum]);
    }
  } 
  return output;
}

Screenshot

Remarks

If you extend the paymentPlan range, to contain the rest of the data as presented in the PAYMENT PLAN sheet, then you will notice that overlapping payments will be summed.

Add the script under Tools>Script editor and press the save button (in your case it's already there).

Example

I've added the solution in your example file.

Perhaps something like this:

=query(A:C;"select C where A=x and B=y")  

with x and y replaced by what it is you wish to select.

Since sight of your spreadsheet, I am not sure of what you would like where but suggest:

1. insert a column immediately to the right of D in Fleet Log and in E2 enter:
   =C2&"|"&D2 and copy down to suit.

2. in Sheet5 Cell C3:

   =iferror(vlookup(C$2&"|"&B3,'Fleet Log'!E:F,2,0),"")

and copy down to suit.

I suspect =VLOOKUP will not work 'backwards' (ie to its left) but INDEX/MATCH may suit. Keep the formula from 1. above but put it in G2 instead (without inserting a column). Then in Sheet5 Cell C3 copied down to suit:

=iferror(Index('Fleet Log'!$E:$E,match(C$2&"|"&$B3,'Fleet Log'!$G:$G,0),""))  

I'm getting curious to know whether this will work/suit!